Inter head coach Roberto Mancini was left to rue Felipe Melo's "stupidity" after his needless foul in the dying minutes saw Lazio claim a 2-1 victory at the Serie A leaders.
The Brazilian midfielder jumped into Sergej Milinkovic-Savic's back with just three minutes left, leaving the referee no other choice but to award Lazio a penalty.
Antonio Candreva - who had scored the opener before Mauro Icardi levelled - saw his initial attempt kept out by Samir Handanovic, but tapped home the rebound to down the table-toppers in their last match before the mid-season break.
Fiorentina and Napoli are now just a point behind Inter, with Juventus and Roma three and four adrift respectively.
"We knew that it would be a difficult game, but we did nothing different than in other games. I am disappointed because we gave away the match," Mancini said at a news conference.
"We levelled the scoring and then threw away the game again. Lazio's winning goal was the result of stupidity.
"We did not play very well, but Lazio did not do much and sometimes you can win even though you play badly.
"We wanted to attack them both down the wings and through the middle, but we attacked very little before the break. We never managed to trouble Lazio's defence.
"We pinned them back inside their own half after the break and did not let them outside of their own area. We must have more patience in situations like this.
"A draw was maybe the right result and we just threw it away, which is obviously bad.
"Do we feel the pressure from our title rivals? The season is still long. We would have taken being top of the league halfway through the season, but we could have had a slightly bigger lead.
"This game has ruined Christmas a bit, but we are still first. Juventus, Napoli, Fiorentina and Roma are the favourites."