Teenage pair Jasper Blackwood and Kit Blomfield both took four cheap wickets as P & S were thrashed by South Wilts.
Replying to the visitors’ 171 all out total, the Division 3 table-toppers were shot out for just 90.
Opening bowler Blackwood removed the top four - of which Matt Benfield’s 13 was the top score - on his way to 4-24.
Then Blomfield - who, like Blackwood, has played for Wiltshire Under-18s this year - mopped up the tail.
He finished with 4-12 from 5.1 overs as P & S lost their last five wickets for nine runs to be skittled inside 25 overs.
Tom Benfield (13) equalled his brother’s score, but no P & S batter scored more as their run of 12 consecutive wins came to a shuddering halt at St James.
Left-arm spinner Keiron Dunstan (3-16) and opening bowler Jake Peach (3-36) had impressed when South Wilts were restricted to 171, Joe Cranch easily the highest scorer with 68.
Despite a first defeat, P & S are still 34 points clear of second-placed Langley Manor - who thrashed Havant 2nds - and enjoy a 35-point lead over third-placed Sway. The top two go up.
One 100 per cent record still firmly intact, though, is one Purbrook won’t wish to keep hold of for much longer.
They took another step towards almost certain relegation to the Hampshire League after a painful 124-run loss to Basingstoke & North Hants 2nds.
It was the cellar dwellers’ 13th successive defeat - against the side immediately above them - and leaves them 75 points adrift of third-bottom Trojans. The bottom two go down, and there are only five weeks remaining to attempt what would be the greatest of all grassroots sporting great escapes.
Replying to Basingstoke’s 202 all out total, Purbrook crashed to 32-4 with Tyler Bradley (5-48) taking three of the wickets.
Matt Donaldson later bagged 3-1 as the final five home wickets clattered for the addition of just two runs - Purbrook all out for 78 in the 22nd over. Ijaz Tarakhail (27) was one of just three batters in double figures.
David Steadman struck a quickfire half-century and took four wickets as Sway kept up hopes of back-to-back promotions with victory over Trojans.
First, he swatted four sixes in a 50-ball 61 as the New Forest club posted 243 all out (Will Crossley 78).
He then removed both Trojans openers on the way to 4-45 - just a fortnight after bagging 5-42 against Purbrook - as the hosts were restricted to 173 (Syed Mahmood 51).
Former Kent bowler Hugh Bernard took his seasonal league tally to 22 - just one behind Steadman - with 3-44.