I noticed the game has a rather nasty but fortunately easily circumvented bug when you buy weapon upgrades from the merchants. Namely, the game checks not the maximum available weapon level you have at the moment, but what level the weapon is currently set to. Thus, for example if you keep the UZA set to level 0, you can keep buying upgrades to level 1 infinitely. Of course, this is only an issue with the weapons that allow you to switch between the different levels in the first place.

edit: I accidentally left the question box ticked when posting. My apologies about that.