Parents have a lot of things to teach children and relatively little time to do it. Most of us are just making it up as we go along, anyway. Everyone has a story about that one time that the kid outsmarted us, like the kid who kept his room clean by living in the hallway, or completely misunderstood the point, or even stories of totally unforeseen circumstances, like the father who auditioned for his daughter's high school play in order to demonstrate that failure is okay and then won a leading role. An askreddit thread has a treasure trove of those stories.
One of my 5 year old twins was still having occasional accidents because she would get so caught up in playing/doing something else that she just wouldn't go and would pee her pants. To combat this we would give her a special prize of some variety when she wouldn't have an accident. This, in turn, caused her twin sister to START having accidents so she could get prizes for not having accidents (even though she was fine on this front beforehand.) We had to rethink our methods. -KyleRichXV
Not a parent, but as a child I noticed my sister was writing her name on the walls when she was drawing on them with crayon. Taking on the role of Helpful Big Sister, I informed her if she was going to graffiti things she shouldn't write her name and give herself away.
A few weeks later, she was carving patterns into the wooden desk in the study and carved my name into it instead. -frozennie
Coworker of mine was trying to teach her kid the "don't talk with your mouth full" rule. Instead, the kid just spits out their food when they want to talk.
Children are the absolute masters of malicious compliance. -MisterCrispy
Read a book that suggested you ask your kid what an appropriate punishment for misbehaving would be and then carry it out. 6 yo son pinched his brother or something, so we asked what an appropriate punishment would be. He said “pluck out my eyeballs and throw me over a cliff?”. We didn't follow through. And stopped reading parenting books. -Mungobrick
Lifehacker has a roundup of the best stories, and you can read them all in the reddit thread. -via Metafilter
Oh yeah, there are more stories in the comments at Lifehacker and Metafilter. Feel free to add yours here.