This is an pretty rare bug, I have had it 3-4 times, it generally happens then I use the gaus rifle I bought from the minutemen lady from old guns, its set up with an long recon scope.

Happens then I hold the gun in scope for an long time trying to nail enemies moving around, pressing any of the mouse buttons, tab or other shotcuts don't work, Esc work so I can reload.

It this an know issue?

Can you try rebinding the scope (aim, default key = RMB) key so that breathe (while in scope default key = Alt) and melee/grenade (also default key = Alt) are set differently? I use F to aim/block myself (and RMB to melee/grenade). That leaves Alt free to be set to breathe (while in scope) and not also be melee/grenade, or anything else that might confuse game. Might be something to try.

Will try, anyway its an good idea as I mess with that button an lot.