While '83 is a shade before I could talk about being there, from what I have heard about the '83 game from a Rita player on the team in the game, is that Rita scored at the end as an MC DB fell down. That pretty much determined the playoff rep at the time as only a conference champ made playoffs at the the time. Final was SR 20-19.
I would say the '06 game comes to mind as far as a favored MC team losing. A great hard-hitting game , MC lost 7-6. Totally controlled the TOP and yards but missed a PAT and Rita won 7-6. The '04-'06 stretch had insanely close games. '04 had the regular season rain game 42-35 and the playoff OT, '05 MC 21-20 and '06 SR 7-6.