Paris Saint-Germain head coach Thomas Tuchel was dismayed after watching his side throw away a two-goal lead in a "catastrophic" second half at Monaco.
The champions looked well on their way to three points in Friday's Ligue 1 encounter at Stade Louis II, as Kylian Mbappe's first-half double put them up 2-0 at half-time.
PSG could have been four goals up at the break, but Moise Kean and Mbappe had efforts ruled out by VAR for offside, which would ultimately prove costly.
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Kevin Volland's second-half brace and a late Cesc Fabregas penalty gave Monaco a stunning 3-2 comeback win, putting them into second place ahead of the weekend's games.