The absolute worst was when dd's language was really devoloping between 18 months and 2 years old. We have been instilling manners since long before she could verbalize them. Of course, one of the things we taught her is when to say thank you. So this completely backfired because for at least 6 months she could not properly pronounce the word "thank". In fact, she wasn't even close. So while that itself isn't a problem and we'd be happy whatever came out so long as she was attempting "thank you", it became a HUGE problem in public.
Let me give you a scenerio. One of many. We go through the checkout line at the grocery store. Dd happily sitting in the cart catches the eye of the checker. The checker says, "would yo like a sticker!?" as they hand one over. We apparently did so well teaching her when to be appreciative and say thank you because she would attempt to say it every chance she got. So, she would happily (and sometimes very loudly) say, "f**k you!".
Dh and I are both non-cussers so she never hears that word... that was just her best attempt at the word "thank". While people usually found it very entertaining I hated having to explain to everyone that my 2 year didn't just cuss them out but, rather she was trying to say, "thank you". She later decided to get lazy like everyone else and drop the "you" and just say, thanks... which then just came out as a simple "f**ks!" greaaaaaaat.
She has long since grown out of that and can very clearly say "thank you" now.
A current one that is probably just as bad: dh always wears a watch. Typically time keepers are called clocks and this is what dd knows them as. She is still unable to pronouce the "l" in clock so you can understand why we now tell her that when she wants to see daddy's watch and we're out and about around others she must make sure to say, "can I see you WATCH daddy", "look at daddy's WATCH", "daddy's WATCH is way cute mama", etc. There were too requests reguarding dh's watch that were sounding so very innappropriate that we had to just take the word "clock" right out of her vocab for the time being!