So I got some new clothes, some of them says “do not tumble dry”. I just had a look at my older clothes, turns out I have been tumble drying them all the time, which I should not have done because some of them says “do not tumble dry”. But it didn’t seem to affect the clothes much.
So is it ok to ignore this warning? What bad can happen if you tumble dry clothes that are not supposed to be dried this way?
When I was a family man I used to tumble dry any old thing on the cool setting, even if it said not to. I can’t recall anything being damaged. Even silk and wool could handle cool, although very slow for wool stuff.
If you tumble dry wet wool (and possibly other fibers), you can end up with felt, and a considerably smaller garment than when you started. Also, sufficiently delicate fabrics, if tumble-dried with garments with zippers in them, can get shredded by the zipper. These would also probably say “hand wash only.”
It’s not so bad, but if you get caught doing something like that and cutting the tags off a mattress, you’re in deep shit.
I never abide by the “hand wash only” tags - I just put them in a mesh bag and machine wash. I’m a rebel that way. However, I do follow the “do not tumble dry” instructions - it’s not that hard to hang or lay flat something to dry and I’ve ruined a number of sweaters, shirts and also a dress by putting them in the dryer (sometimes accidentally.) Some fabrics really cannot handle it - not worth the risk to me. 
As far as I know, the washing instructions are for the best results, so you don’t ruin your clothes. If you’ve been tumble drying and you are okay with the results, I wouldn’t worry about it. In spite of what Rhthmdvl says, no one is going to come to your house and arrest you if you don’t abide by laundry instructions.
I don’t listen to those instructions much, either - I wash and dry everything the same (regular wash, warm/cold water, hang to dry - I hang to dry just to save electricity, and because all my pants shrink in length if I tumble dry them).
What happens? They shrink?
Some fabrics will shrink quite a bit, and turn from soft to stiff as a board.
Some things you can get away with it, some things you can’t. Generally, I’ve found the more you pay for something, the more likely you can get away with it, because at least the fibers all tend to shrink at the same rate. You may end up with a smaller garment, but it’s still going to scale correctly. Cheap clothes sometimes use fabric where the warp and weft are different fibers, so it may shrink on the vertical axis but not the horizontal or vice versa. Worst of all is when they use *thread *that shrinks differently than the fabric it’s holding together - then you just end up with warped, puckered clothing that’s entirely lost its shape.
But my personal philosophy is that life is too short to go to the dry cleaners more than twice a year. So almost everything gets washed/dried. If it comes out fine, then I can ignore the label. If it doesn’t, it gets turned into rags. I have to really really love something to give it special handling.
Man, you’re not kidding. I was given some woman’s size M Ralph Lauren sweater at a clothing swap to give to my 7 year old daughter, because it had shrunk when someone accidentally put it into the dryer. Put it on her and it was cute in a slightly-oversized waif kind of way. We rolled the sleeves up a bit, leggings underneath, and it was cute.
I figured since it had been through the wash/dryer once, it had done all the shrinking it was going to do, plus, see above for my laundry philosophy, so I machine washed and dried it.
Hmm…we wore this with leggings last time? Maybe tights and a skirt would be more appropriate. Kiddo must be growing.
Repeat a few times.
Kiddo now has a Ralph Lauren wool felt sweater for her doll.
Well, I toss “dry clean only” stuff in the wash with everything else, so you probably don’t want my thoughts here.
My dryer doesn’t even have a cool setting, or a temperature setting of any kind. And everything goes in the dryer. So you definitely don’t want my opinion!