Self immolation
Looking behind you when being chased.
RUN, DAMMIT!!!
Thinking that you can just pick up that feral cat rubbing against the steps with your hands and put it in a carrier. Bonus points if you tried it yesterday and got your leg shredded.
Also, if a cat tries to jump over a chainlink fence and gets its toe stuck and is hanging upside down squalling and hissing, its never a good idea to run past your thick leather gloves to use bare hands to save said cat. (At least I only had to learn that lesson once.)
Better yet, sabotaging his ever-present condom with the idea that once he learns that (surprise surprise) You’re Pregnant!!, he’ll give in and marry you.
Second, not stopping to wonder why he always insists on using a condom, and whether it’s totally a question of contraception, vs. other reasons.
And why said guy goes to Hong Kong every six months to get a blood test . . .
Why yes, I am thinking of someone specifically, why do you ask?
Giving a guy unsolicited workout advice in the gym.
I know someone who did this. He didn’t get it, then drank beer on the way home. No, he didn’t get caught. 
Adopting a Jack Russell/Chihuahua mix, commonly referred to as a Jack-Chi. Amazingly intelligent and agile dog with a severe attitude problem and more energy than a kindergarten class with a piñata.
Related: attempting to remove a lead from the collar of a very angry dog. Got my first dog bite this way! I still have his teef marks. Pics on request. 
Keeping Ben-Gay and K-Y in the same nightstand drawer.
Googling something that people warn you not to Google is always a bad idea. 
Yeah, I wondered about that one, myself. Really? That’s your big day?
In that vein, lipping off to Judge Judy is always a bad idea, but always entertaining for onlookers.
Also, saying things like, “It’s quiet out here,” or, “Wow, I’m sure glad THAT’S over, and we’re in the clear now!” are never a good idea.
Looking in the barrel of ANYTHING to see why it’s plugged - not recommended. Sometimes you’ll get a faceful of crap, and sometimes you’ll get your head blown off.
Hitting send when you still have some doubt.
Using a newfangled technology to set up liaisons with your mistress, while you’re still not clear what the actual purpose of that technology is supposed to be.
Troy Dungan, WFAA-TV meteorologist, wore bow ties on the air almost all the time during his run there (Thanksgiving was the exception), and I thought he looked good in them (this based off some video clips that I saw of him, which have since been removed).
Always a bad idea - answering a female who has just asked, “Do you like fat women”. Sir Harry Flashman had some trouble with this one.
Not even then. Asking that question implies that you think she is fat. In the words of Dave Barry, “Never ask a woman if she’s pregnant unless you can currently see a baby emerging from her”.
Funny. I was 16 in 1995.
You all are impressed, right?
Calling J.G. Wentworth 'cause you “need cash now”.
Cigarettes
Your mom
Glad to see this one said quickly. I have had one guy argue with me that it’s worth it because the sex is good. Well, first of all, crazy doesn’t necessarily mean the sex will be good and good sex can be gotten without the crazy. Second, if she does happen to get pregnant or you somehow get suckered into a relationship, you’ll agree then.
I would generally agree as it usually just get’s a “I KNOW WHAT I’M DOING!” response. I have, however, done so when someone was immentently going to seriously injure themselves or damage the equipment. You know those guys trying to squat WAY too much weight, on their toes with their back arched and locking their knees at the top? Or that guy who somehow imagined that tricep machine was for biceps if you sat in it backwards and contorted your body in a weird and obviously uncomfortable and dangerous way. That’s about it though.
For my own contribution, using the internet to diagnose and treat yourself for a serious illness. Ditto for law or any other profession that is usually practiced by people who have years of schooling.
Buying anything solely because it’s claimed to have been “clinically proven”.