Leigh Griffiths became just the eighth player in Celtic's history to score 40 goals in a single season on Wednesday, but he could not prevent the Scottish Premiership champions from losing 2-1 away to St Johnstone.
The striker opened the scoring for an inexperienced Bhoys side in the 53rd minute at McDiarmid Park, dancing into the area and finishing past Alexander Clark after being played in by Stefan Johansen.
But the Saints responded just three minutes later through Steven MacLean, who pounced when Johansen failed to control the ball inside the area, slotting home from close range to make it 1-1.
St Johnstone snatched what proved to be the winner with 13 minutes to go, substitute Graham Cummins taking advantage of a mix-up between Celtic defender Efe Ambrose and his goalkeeper Logan Bailly to fire into an unguarded net.
Kilmarnock, meanwhile, already consigned to a relegation play-off, drew 1-1 at Dundee, while Inverness beat 10-man Hamilton 1-0 away from home.
Elsewhere on Wednesday, Stephen Pearson claimed a brace as Motherwell won 3-1 at Ross County.