I don't know why this was downvoted. I am experiencing the same thing. I am also in a multiplayer game my friend is hosting.
I used a hotkey to build a power pole after holding the rebar gun in my hand. I saw my cursor switch to the pole, but when I clicked I instead shot the rebar gun. Hitting R would reload the gun, despite me still apparently holding the power pole.
I am unable to open the Q menu anymore. Hitting Q or escape does nothing and does not clear the item I am holding (either gun or power pole). I am thus unable to interact with any storage or building contents since my hand is full.
I can open the tab menu and unequip the gun. This allows me to build the power pole (since I no longer can shoot the gun). Equipping other items exhibits similar behavior as the gun (when I equip the chainsaw, the sound is continually running and clicking results in the use chainsaw animation).
When I have unequipped the gun, I can open the builder menu by right clicking (while still holding the power pole in my hands). I can interact with the menu normally at this point, but Q will not close it - only Escape. I can use this to change the item on my cursor, but this leaves me in the same state of forever holding that new item.
This persists across game restarts.