Stuttering Lazio sunk by Chievo
Roberto Inglese's 90th-minute strike, a scrappy close-range effort, saw the northern minnows snatch all three points at the end of an uninspiring contest at the Stadio Olimpico.
Having coped relatively comfortably with Lazio's territorial domination, the visitors held out through a frantic five minutes of added-on time to claim a win that bolsters the buffer between them and the relegation battle.
For Lazio it was a second successive defeat after Simone Inzaghi's squad's tame surrender at leaders Juventus last weekend.
Inter Milan, who were due to entertain Pescara later on Saturday evening, now have the opportunity to leapfrog Lazio into fourth place.
Top three Juve, Roma and Napoli are all in action on Sunday.
Saturday's match was preceded by a scrupulously-observed minute's silence in memories of the victims of an avalanche which left 29 people dead in the Hotel Rigopiano in the mountains of central Italy.