I just found a similar bug while I was playing Level 3 (Avernus Annex). When I got the chaingun in the first secret area, the chaingun's pickup sprite turned into the same sprite as when the chaingun is held in your hand. It fired more rapidly than normal, and firing the shotgun or punching with the fist also fired the faster chaingun's bullets at the same time!
I've only been able to pull this off twice. I've tried for a couple hours trying to replicate it a 3rd time with no luck. By the way, I was playing on a slightly-modified version I made where the E and D keys affect Poomguy and the monsters' gravitational forces.
I think this bug has to do with object-oriented programming anomalies. Retro Game Mechanics Explained made a video some time ago going further in-depth about what I mean. But I don't have time to check so sorry.