If you place an upgradeable ghost item (say, an assembling machine level 1), shoot the shotgun, and then use the upgrade planner to upgrade the ghost while the projectile is in flight, it places the upgraded item (an assembling machine level 2 or 3). Tested on Factorio 2.0.76 (base only) with Blueprint Shotgun 0.2.18.

A video example and save with a minimal reproducible example are available, but the GitHub has issues and discussions turned off, so let me know if you want me to post them somewhere.

Dangit, I had this typed out and I was going through tabs and never sent it:

Wow, that is a crazy exploit. It makes sense with how I'm doing things, but I'm surprised nobody ever found (or reported) it until now.
Thanks for the report, fixed in 0.2.19

I ran into the same bug today. I was upgrading belts from yellow to red and was confused where the one random blue belt came from. I was playing space age w/ some other mods but can replicate in Factorio 2.1.14 with Blueprint Shotgun 0.3.3, same process to cause the bug.