Funny thing about the N64: register R0 is always 0. It can never change.
Turns out some games <cough>Forsaken64</cough> seem to think that's a great place to dump error codes.
800029B8, 800068D8, 80006A6C, 80007410, 800080C4, 80008150, 80008788, 800087F0, 80009D0C, 80009D44, 80009DC0, 80009E60, 80009F58, 8000A02C, 8000A118, 8000BBA8:
SW V0,0000 (R0)
Would be slightly less idiotic if they didn't test for error codes after each return, or if some of these routines weren't dedicated to testing for this error code.