Can someone tell me why this happened? Right in the middle you can see the pinkish discoloration of the body filler showing through the paint job.

 

Inadequate primer, needed an extra coat, inadequate paint thickness as well likely. That's why I always check thickness on a few places where the taped off sections are getting full overspray.

Then I can stick a paint depth gauge in it and read how many mils thick I'm shooting. When I used to spray every day it wasn't a big deal though, I knew the equipment enough that it wasn't really necessary.