Reigning French champions Castres lost at home in the last round of the Top 14 regular season on Saturday to surrender a playoff place to Montpellier.
Castres, who started the day in fifth, suffered a third straight home defeat as they succumbed 25-16 to Toulon.
"We didn't see this defeat coming, maybe that's the problem," said coach Christophe Urios.
Montpellier leapfrogged into the last playoff spot as they eked out a 28-27 victory away to Clermont, who were already assured of a first-round bye.
"I am satisfied because we have pulled off something special," said Montpellier coach Vern Cotter.
La Rochelle, who opened the day in sixth, the last playoff spot, crushed Bordeaux Begles 81-12 to climb to fifth.
Against Toulon, Castres jumped to an early eight-point lead after fly-half Benjamin Urdapilleta kicked a penalty and then scored a 19th-minute try. But the talented Toulon squad, having long ago wrecked their own season, then destroyed that of Castres with two tries by fullback Daniel Ikpefan, sandwiched around one by Anthony Belleau, who also kicked 10 points.
The Castres collapse meant an early end to the Urios era. The coach had already announced that, a year after guiding the club to an unexpected title, he would be leaving to manage Bordeaux-Begles.
"Honestly, I didn't think it would end tonight," Urios said.
"We have been missing commitment and aggressiveness and made individual mistakes," he added. "We saw a team of Castres without springs, worn out."
"Now I want to go on holiday, rest and quickly switch to another challenge with Bordeaux. But it's true it's ugly to end up like this. I'm sad, it's a waste."
At the end of February, Montpellier sat ninth and 13 points off a playoff spot, but finished with eight victories in the last nine games.
On Saturday they were without veteran forwards Louis Picamoles, Bismarck Du Plessis and Fulgence Ouedraogo but tries by winger Nemani Nadolo and hooker Youri Delhommel three minutes apart put them eight points ahead with 16 minutes to play.
Substitute hooker John Ulugia replied to close the gap to one point, but after fullback Setariki Tuicuvu touched down with four minutes to play, video review showed that Ulugia had knocked on in the build-up.
"We lost a lot of matches by narrow margins and now it's switching in our favor," said Cotter. "Why not imagine extending the run to the final!"
Montpellier captain Benoit Paillaugue said the team was not keeping up with scores from other matches "so as not to pollute our heads".
"We thought maybe it was between us and La Rochelle but, in the end, it was Toulon who saved us."
Montpellier will visit third-place Lyon, who won 24-23 at Grenoble, in the quarter-finals.
La Rochelle, who ran in 12 tries on Saturday, return to the playoffs and will play fourth-place Racing 92, who won 35-3 at Agen.
The two runaway leaders, Toulouse and Clermont, will have byes next weekend.
Toulouse ended the regular season with a 47-7 home romp over last-place Perpignan to finish on 98 points, a Top 14 record.
They also scored seven tries to reach 102, yet another record. Clermont, however, kept their grip on the record for most total points in a season. They set the old record, 800, two seasons ago. This time Toulouse scored 820 but Clermont managed 828.
Playoff schedule (all times GMT)
Final (Stade de France)