Well, to be fair to Mom, Cecil is on her side. 
Back when my grandparents lived on the farm, they didn’t have indoor plumbing, so taking a bath meant heating up water on the (wood) stove and filling a steel tub in the kitchen. My grandfather was washing up during a thunderstorm when lightning hit the telephone line, and jumped from the phone to the tub.
So, not only was I not supposed to talk on the phone when it was storming, I wasn’t supposed to take a bath when it was storming.
Also, anything and everything that could put your eye out.
Thunderstorms. You cannot talk on the phone or take a bath during one. These are perhaps more grounded (heh!) in reality than the “cookie dough gives you worms” things (which my mom also said), but, in Mom’s world, a deadly electrical shock during a storm was more likely than not.
My mom told me one time about her mother making her (Mom) and her sister (my aunt) sit on a trunk, with their knees tucked up so their feet weren’t touching the ground, in a windowless hallway during thunderstorms.
One of my sisters is absolutely terrified of storms. Can you imagine? How did that happen?!?
ETA: I missed Alice the Goon’s mention of this upthread!
Running on stairs has broken the neck of every kid who’s ever even considered it.
Sitting closer than 6 feet away from the TV will cause instant radiation death.
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NEVER stick your arm or any other limb out a moving vehicle. Because that’s the EXACT moment a Bus will come from the opposite lane and blow your limb clean off. Don’t believe me, what do you think happened to little Billy down the street? (No one ever MOVED away in our neighborhood, they just became moral examples…)
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Never stick your head underneath the seat of the bus, especially while it’s moving, because if the bus slams the brakes, your neck will be snapped, that’s how your great aunt _____ died. And why no one’s heard of her. We’re all too ashamed. (that one ACTUALLY freaked me out for years. Even now, I won’t do it. I’m not fucking sticking my head under a bus when it’s moving, because it might actually be plausible. Never mind the acrobatics I’d have to pull in order to do it now with my height and weight, but still- that’s how my unknown aunt passed away. And was forgotten by every other relative. The Unspoken Shame). Now when I ask my Mom about it, she swears she told me it was someone else’s aunt and not mine. But still. <_<
-Sometimes if my mom really wanted to add emphasis, the story would change such that it also happened to a little boy (Billy’s brother from down the way maybe?) who did it too, and the bus slammed so hard that his head CLEAN POPPED OFF. So yeah. Either you’ll break your neck or get decapitated. (O_O) Teach that to your 5 year old. I dare ya… -
Never use Public Restrooms. Just… you don’t wanna do it. Its… unhygienic.
Even now my mom complains that I’m so dirty because since going to college, I’ve started using public restrooms whenever I have to go. I’m not holding it in anymore for 3-4 hours! Even now, I have a wierd feeling about public restrooms, and I’ll use those little paper seats even though I know they don’t really do anything (it took me forever to get weaned off of breaking the toliet paper into little strips to cover the seat), and I still have a shy bladder. “Because Urinals… are a thousand times worse. There could be pedophiles. Just staring and waiting to prey on little boys. And that’s where they’ll kidnap you.”
So even though I’m in my 20s, I still use the stall even if it’s just to urinate (if there’s a line, I’ll wait for those who need it to go first. After all I’ve trained myself to hold it!).
Thanks Mom. I love you too. (-_-)
Ah, the archetype of all Dangerous Activities…
Running with scissors.
Listening to rock music. In the '80s the whole lot of 'em were Satan-worshiping, backwards-message-recording, animal-dismembering, screaming-at-the-top-of-their-lungs brain washers whose only goal was to lead America’s youth to ruin.
My mom fell for that crap hook, line and sinker. But to give her credit, she feels pretty dumb about it today. 
Not covering a mirror when there is an electrical storm will mean you will be fried.
Toilet seats were the Most Dangerous Thing on Earth according to my mum. One should never actually sit on the thing, but squat above it allowing your pee to flow free…and thereby avoid all sorts of nasty things to crawl up and infest yer’ nether regions.
I realised very early in life that the EFFORT involved in following her instructions was probably not worth the reward…I’ve been parking my arse on dunny seats for years now and have never been graced with an infection or an infestation.
Toilet seats are the great equalisers. 
Q.E.D., not Cecil.
As the father of a 5 year old, I’m quite familiar with paranoia. In 20 years, my son may well be on the SDMB telling people about the crazy, panicky Dad who wouldn’t let him have any fun, and got worried about the silliest things imaginable.
I may even DESERVE it!
But I suspect he WON’T remember all the insane, life-threatening things he’s wanted to do that WOULD have killed him had psycho Dad not been around to stop him.
Also, bear in mind that many of the items I’ve seen posted in this thread, while silly and outdated, were NOT made up by our Moms! My Mom didn’t decide, out of nowhere, that I had to wait an hour after eating to go swimming. She was following the conventional wisdom, something that all the “experts” agreed on, something that appeared in all the newspapers, magazines and TV news shows.
If, in the end, that wasn’t REALLY something to be too concerned about, blame the media, not your Mom.
Sitting on cement.
You’ll get hemorroids.
(I have no idea where she got this one)
Staying out even one minute past dark will immediately result in winding up dead in the gutter. It’s like they thought we were reverse vampires or something.
33 posts, and nobody has yet mentioned :
BB guns.
and for fun, here’s a video that sums it all up.
Well-known music, with well-known words…
My mother was convinced that an extinguished cigarette could magically burst into flames 3 days later and burn the house down.
Don’t make faces. It might stick. Especially if you cross your eyes.
Riding double on a bicycle was strictly prohibited.
Go out of the house with wet hair and you’ll “catch your death.”
I better think twice about kissing boys with bad teeth. They’re contagious.
There’s an urban legend that driving barefoot is illegal. The Barefooter Society checked with every state and nobody cares. (I do it all the time)
(Also, neither OSHA nor the Health Department can’t keep you from going into stores or restaurants while barefoot, since they regulate employees. The business, of course, can make any policies they want regarding customers.)
A ‘funded’ government, populated by people elected by the uneducated masses.
Oh, according to Mom…
Going to a movie without a chaperone, because if you sit in a dark theater alone you’re sure to be molested, raped, robbed, sold for parts, etc.
Fortunately, being accompanied by anyone, even one’s 5-year-old sister, causes all molesters, rapists, robbers, flesh merchants, etc. to steer clear.
Doing anything unsavoury on the internet.
Giving away personal details on the internet.
Talking to anyone on the internet.
Using the internet.
The existence of the internet.
My immigrant grandmother continually told me and all my cousins that we should never sit on:
- A cold surface
- Cement
- A hot surface
- Rocks
- Uneven surfaces
If we sat on a cold surface, we would freeze our ovaries. (The boy cousins did not get that particular lecture.) If we sat on any of the other surfaces, we would get hemorrhoids. I think my granny would have liked it if we never, ever sat anywhere but a nice chair.