Okay, I’ve dropped an egg on another one and one cracked... but hey, neither were fertilized 
Mistake 1 (pre incubation)
-What did you do?
Buy local hatching eggs during a heat wave
-What were the consequences?
5/8 were not fertile
-Did you fix it? How?
uh...
-What did you learn?
Be aware of your weather/conditions when buying hatching eggs whether it’s online or local. Don’t be shy to request the order to be delayed until better weather is abound. Local eggs are not going to have a higher hatch rate than shipped ones if they were laid in less than ideal conditions.
Mistake 2 (post incubation)
-What did you do?
Leave weak chick that I was nursing with broody at night.
-What were the consequences?
broody left it out in the cold and it died
-Did you fix it? How?
Currently studying to be a necromancer
-What did you learn?
Broodies do take care of chicks well, but sometimes if a chick can’t make it under mom by themselves, then they will be ignored and left to natural selection. I don’t know whether it’s my specific broody, but I witnessed a similar scene with my current hatch. One chick that just hatched didn’t quite dry up well, and somehow ended up kicked out from under mom. It was chirping up a storm, which would normally send the broody into distress, but the broody was completely calm and ignoring the “weak” chick.
I ended up giving the chick a long warm bath, after which it managed to fluff out properly, and mom then accepted it without issue.

Mistake 1 (pre incubation)
-What did you do?
Buy local hatching eggs during a heat wave
-What were the consequences?
5/8 were not fertile
-Did you fix it? How?
uh...
-What did you learn?
Be aware of your weather/conditions when buying hatching eggs whether it’s online or local. Don’t be shy to request the order to be delayed until better weather is abound. Local eggs are not going to have a higher hatch rate than shipped ones if they were laid in less than ideal conditions.
Mistake 2 (post incubation)
-What did you do?
Leave weak chick that I was nursing with broody at night.
-What were the consequences?
broody left it out in the cold and it died
-Did you fix it? How?
Currently studying to be a necromancer
-What did you learn?
Broodies do take care of chicks well, but sometimes if a chick can’t make it under mom by themselves, then they will be ignored and left to natural selection. I don’t know whether it’s my specific broody, but I witnessed a similar scene with my current hatch. One chick that just hatched didn’t quite dry up well, and somehow ended up kicked out from under mom. It was chirping up a storm, which would normally send the broody into distress, but the broody was completely calm and ignoring the “weak” chick.
I ended up giving the chick a long warm bath, after which it managed to fluff out properly, and mom then accepted it without issue.