Leinster missed the chance to extend their lead at the top of the United Rugby Championship after suffering a 44-12 defeat to the Emirates Lions at Emirates Airline Park.
The Irish province, which last week booked its place in the semi-finals of the Investec Champions Cup after eliminating defending champions La Rochelle, have won 11 of their last 12 matches in the competition.
However, after making 13 changes, Leo Cullen's side were beaten inside the first 15 minutes as the Three Lions tried to put them on a winning path, boosting their playoff hopes.
Leinster now lead just one point ahead of Glasgow with four rounds remaining as the Lions climb into the top eight.
The Lions got off to a bright start and got their first try inside 40 seconds when Richard Criel and JC Pretorius combined to send Morne van den Berg under the posts, with Sanele Nohamba scoring before converting a penalty seven minutes later.
A brilliant team effort saw the Lions score again in the 10th minute, with Frank Horn, Van den Berg and Marius Low all involved before the latter got going but Nohamba was unable to add the extras.
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And it went from bad to worse for Leinster three minutes later as Nohamba took advantage of a loose ball before putting Kwan Horne ahead, which the former converted for a 22-0 lead.
Leinster duo Liam Turner and Cormac Foley had attempts disallowed for a strike and a forward pass respectively either side of the break before a Nohamba penalty increased the Lions' lead.
The Irish side finally got the win in the 53rd minute when Ciaran Frawley latched onto a powerful kick from Harry Byrne, who added the extras.
But moments later the Lions got the bonus point when Emmanuel Chitoka went over the bar after a counter-attack from Leinster, but Nohamba was unable to convert.
Frawley had his second try after a brilliant pass from Leinster but Byrne missed his kick to leave the score 30-12.
Chitoka and Frankie Horn added further tries in the final five minutes, both scored by Jordan Hendricksie, to cap an excellent day for the Lions.
