Definitely get a close look at the wiring to the switches on all the drop target banks. Make sure you don't have any diodes or wiring touching any adjacent wiring or anything. If everything was working fine until you were working on the drop targets, that's gotta be where you'll find the solution. If you can't come up with anything, post some pics of the switches on the drop target banks and I'll take a look and see if anything looks amiss.

Sure, it's possible. I doubt it though, too coincidental. I just find in my own experience that when i work on something and it doesn't work afterward...90+% of the time it's because i made a mistake. That kind of wacky behaviour definitely sounds like wires could be touching or something like that. I would be surprised if a new board (that was working fine before) would spontaneously develop a problem. Start with the simple stuff! Like i said, send pics if you're unsure. I'm thinking maybe one of the switches wiring terminals may have gotten bent when you were putting on the decals.