^ tbf the "Mattel rifle's" stopgap intro during 'Nam was a trainwreck of multiple different design quirks (most notably the specific form of gas operation chosen and the cartridge primer then in use) and FUBARed communication (namely the troops getting the idea the gun required less maintenance and cleaning when the exact opposite was true) running headlong into the harsh conditions of jungle warfare. While the technical issues were ironed out soon enough it took a long time to regain the lost confidence, not particularly helped by unavoidable association with the widespread public disillusionment with the whole conflict (PROTIP: using conscripts in expeditionary warfare is a Very Bad Idea)...
For modern weapons that were legitimately terrible upon introduction look no further than the justly notorious L85. Took the better part of a decade and Heckler & Koch assistance to fix, too...