Luca Caldirola returned to haunt Inter as Monza snatched a stunning 1-0 win at San Siro, a result that means Simone Inzaghi's problems are deepening.
Defender Caldirola started his playing days at Inter but did not make the grade as a first-team regular, going on to have a journeyman career. However, a powerful header from the 32-year-old snatched all three points for Monza on Saturday, as Inter fluffed their big chance to go fourth in the table.
Milan's draw with Bologna earlier in the day had opened up that opportunity, but Inter's dismal run of form continued as Caldirola claimed the spotlight.
It is now one point from the last five league games for Inter, a desperate run at this crucial stage of the season.
Inter striker Romelu Lukaku headed wide from an excellent position early on, wasting a big chance, and there were to be several of those for the home side.
Joaquin Correa then combined neatly with Henrikh Mkhitaryan and Lukaku as he carved out a route into the Monza penalty area, but the Argentine forward was denied by Michele Di Gregorio's save.
It was Correa who was providing much of the early Inter threat, but the first half proved a largely drab affair.
Monza's Carlos Augusto fired into the side-netting as the visitors posed an early threat after the break, with Inter losing defender Stefan de Vrij to injury.
Nicolo Rovella also threatened for Monza with a long-range effort before Inter's Lukaku had a header clawed away by Di Gregorio.
Correa sent a shot narrowly wide, and Lukaku fired fractionally over the bar as the hosts pushed for a breakthrough, yet Dany Mota went equally close for Monza with a left-footed strike that went just too high.
Monza stunned their hosts in the 78th minute when Caldirola stomped through a crowd to head in Patrick Ciurria's left-wing corner, and Inter could not find a way back to level terms.