Climbed up a ladder (green sentry tower, in southeast corner of Pavlovo military base, by concrete wall, behind barracks with shipping crates), with Vaiga shotgun in hand, then climbed down, equipped combat knife (hotkey slot 4) to kill zombie at the bottom of ladder. From my perspective, the knife sounds and animations were playing, but no hits were connecting, but with each knife attack, my friend reported hearing shotgun shots, (which I neither heard, while using my knife, nor saw any shot impacts). I was soon swarmed by zombies (meaning the shots were registered on the server, but on my client I was only seeing my knife attacks), and I could not put away my knife to use my bandages (hotkey slot 5). After using the hotkey slots several times to swap between items, I noticed that my Vaiga shotgun on my back had disappeared (hotkey slot 2) and I was able to use my bandages. I can only assume that when I equipped my knife, on my side, I had my knife in my hands, but on the server side, I had my shotgun, and with each knife swing, I was also shooting the shotgun. The issue only went away after the shotgun deleted itself somehow.