Munster are out of the Investec Champions Cup.
Christian Ambadiang's late try was enough to give Castres a 31-29 victory at Thomond Park.
Despite picking up a try scoring bonus point, Clayton McMillan's side can't progress to the knock out stages.
They will now head to the Challenge Cup thanks to results elsewhere in the group.
Leinster are counting the cost after coming from behind to beat Bayonne.
Four players left the pitch early as they won by 9 points and kept the unbeaten streak alive.
In the Challenge Cup, Connacht stormed to a big victory over Montauban in Galway.
The hosts ran in 10 tries as they made it two wins from 4 with a 68-14 win at Dexcom Stadium.
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The Wolfhounds can return to the top of the Celtic Challenge table later.
The Irish side are 3 from 3 so far ahead of their meeting with the Brython Thunder at noon.
Wales' Gwalia Lightning are the team to catch so far and have a two point lead at the top of the table.
Clovers can also close the gap when they welcome the Glasgow Warriors at 3pm.
