Timeslot moves that killed or almost killed a show

[KurtfromPitts]

When ABC moved The Partridge Family from Friday to the death slot on Saturdays opposite All In The Family in 1973.

[megamanj2004]

As for game shows:

When CBS moved Press Your Luck from its 9:30AM timeslot against NBC's Sale of the Century to the late afternoon timeslot of 3PM (which replaced Mark Goodson's Body Language) to make way for the return of Card Sharks, PYL was done for in September of 1986.

Even when NBC moved Sale of the Century from its 9:30AM timeslot to 9AM against a waning $25,000 Pyramid, Sale still did reasonably well, even with the bonus round change from Winner's Board to the Winner's Big Money Game. But that glory wouldn't last very long once CBS returned Family Feud back to the air on July 4, 1988. The timeslot change, sagging ratings plus NBC's game show purging did in Sale in 1989.

[megamanj2004]

I got another one:

AfterM*A*S*H - in its 1st season, it surprisingly finished out in the Top 10 shows during its initial Monday timeslot (the same timeslot as M*A*S*H).

When a promo for Season 2 came for their move from Mondays to Tuesdays, where they made a mockery of cutting Mr. T's signature mohawk and their claim that they'll beat Mr. T and The A-Team in the ratings, the joke ended up being on AfterM*A*S*H. Not only did Mr. T and The A-Team pummeled AfterM*A*S*H and E/R (for that matter), but AfterM*A*S*H ended up getting canned 9 episodes into the 2nd season and cemented its place as one of the worst shows in TV history.