The Japanese Type 26 Revolver was almost (but not quite) as bad as the Type 94 Nambu- while you couldn’t fire the Type 26 without pulling the trigger, the cylinders didn’t always align with the barrel. This is a bad thing, in case anyone was wondering.
The Canadian Ross Rifle wasn’t a bad rifle (they were very accurate and made excellent range rifles), but it was patently unsuitable for use as a general service rifle, especially in WWI. They jammed a lot in the mud of the trenches, and because of the straight-pull design, if they weren’t reassembled properly, the bolt could end up flying straight back into the firer’s face when the gun was discharged. This, too, is a bad thing.
The Mauser C96 “Broomhandle” was itself an excellent handgun, but during the Spanish Civil War a “Schnellfeur” (Full Automatic) version was developed, which had an insanely high rate of fire (something like 600 or 800 rounds a minute). On paper, this sounds all fine and dandy, but in reality, the Schnellfeur would empty its magazine in literally 1 second and very few of the bullets were likely to be anywhere near their intended target. This is, unsurprisingly, a less than desirable state of affairs.
The French also created a pistol in a calibre that didn’t exist (7.65mm Longue) during WWI as a result of misreading information from Britain and the US, who were working on a conversion device for their Lee-Enfield and Springfield rifles to make them self-loaders. The French caught wind of the non-existent 7.65mm Long cartridges (invented to confuse the Germans), made a pistol in the calibre, then looked very silly when the British and US asked them why they were making a gun for which ammunition didn’t exist. Needless to say, the French promptly made the cartridge as well and then quietly shelved it all once the ink on the Armistice was dry.
Some of the stuff the Germans knocked together in the closing stages of WWII would also qualify (Some of them were basically Zip Guns in 7.92x57mm Mauser), but they generally get a pass as being the products of desperate times, unlike most other “Worst Firearms Ever”.
The SA-80 also gets an honourable mention for initially being a spectacular piece of crap (with magazines falling out of the rifle while it was being fired and so forth), but the British recently sent them off to H&K for rearsenalling and that seems to have ironed out most of the issues and turned it into a decent gun after all.