Nothing serious but when I was a kid my doctor regularly forgot to mark my blood samples with my age, resulting in me having to take Vitamin supplements on and off because the values flip-flopped between “way too low (for an adult)” and “way too high (for a 10 year old kid)”
Luckily my parents wizened up to that after a year and we switched doctors, was not a nice experience but still on the more harmless side of things
Edit: I do have a really messed up one. Back when I was a little toddler crawling on all fours my parents gave me milk in glass bottles, which is a good idea because plastics are obviously bad until you consider toddler behaviour. Well things inevitably happened and toddler me dropped the bottle while sitting on a bench. Toddler me then had nothing better to do than to follow the bottles course and fall into the glass pile. Parents rushed me to a child surgeon, he removed “all” glass pieces from my elbow. A few days after the OP the wound starts watering badly, so my parents go back to the doc. Nope he says, all the glass is removed. Wound doesn’t get better so my parents go to a normal surgeon. Dude looks at the wound, cancels his plans and essentially emergency operates the wound because lo and behold there were still glass fragments in my elbow. Scars being what scars are I now have a ~10 cm and a ~5cm scar stretching across my left elbow. Guess the only good part about it is that I was too young to remember that shitshow.