If you use pistol with your main weapon on your back and drop the pistol from your inventory while you are CROUCHED, your character will be stuck on pistol holding animation and will get unstucked by switching stance from crouch to something else or by vaulting object for example.
Also, you can fire off the main weapon wildly in air while it's still held in your back, possibly hitting some birds or occasional Santa Claus in process.
#1: Play any mission with soldier that has main weapon and a sidearm.
#2: Equip the sidearm, switch to crouch stance, open inventory.
#3: Drop the sidearm in any possible way and close the inventory.
You are now stuck in this stance and cannot move without manually getting unstucked by performing some other action such as switching stance so that your character automatically switches back to main weapon.
Additional bug related to this one:
If you attempt to open your inventory when crouched with sidearm on safe mode, (not aiming, pointed towards ground) your character will first go to prone, and then back to crouch stance before opening the inventory.