When shooting mobs in certain areas (mainly the bigger creatures), the weapon will both fail to do damage and generate an error on the log per hit. This also causes a TPS drop, barely noticeable sometimes but intensity varies from creature to creature and weapon to weapon.
TF2 NPCs don't seem to be affected by this, they can shoot freely everywhere they want and nothing happens, they deal damage without issues.

Same issue happens with some Wyrms of Nyrus mobs.

Le log: https://pastebin.com/s3ezZqGT