Only Leicester, at the zenith of their power under the Dean Richards-Martin Johnson axis, have managed that, at Munster's expense in Cardiff ten years ago.
Clermont are no slouches either when it comes to a winning streak. Theirs is not as long as Leinster's but impressive just the same - one defeat in 15, at Biarritz in the Top 14 on March 31 and then by a single point.
At the Stade Chaban-Delmas on Sunday, the Michelin Men will be attempting to reach their first Heineken final by starting where they left off against Saracens in the quarters, assuming Leinster allow them. The stadium in Bordeaux, named after Jacques Chaban-Delmas, French Prime Minister for three years until 1972 after a rugby career which brought him one international appearance in 1945, has already fallen to the Irish.
Mick Galwey's Munster went there twelve years ago for an equally imposing semi-final against Toulouse and proceeded to win against all odds, 31-25. By the time Leinster get to grips with a Clermont team considered by many to be the best in France, Ulster or Edinburgh will be awaiting the winner in the final.
How Edinburgh have soared to unprecedented heights while plumbing the depths of the RaboDirect PRO12 is by some distance the mystery of the season. One of the Scottish capital's most famous rugby citizens is as puzzled as anyone else.
Andy Irvine, the distinguished Scottish Lion, has followed their progress with added interest given his position as chairman of the RaboDirect PRO12. Even he admits to defeat when it comes to understanding how Edinburgh can do the double over Racing Métro and London Irish, then lose their next five PRO12 matches on the bounce.
'To be honest, it's very hard to explain,' Irvine says. 'It beats me. The Racing Métro game where they came from a long way behind to win 48-47 was nothing short of unbelievable. Then to go out against London Irish when they had not only to win the game but get the bonus point as well to qualify and achieve it was another terrific performance.
'For some reason, they seem to pull it off in Europe but not in the League. All the odds for the semi-final in Dublin will be against them because Ulster are the form team. Any team capable of beating Munster at Thomond Park as they did in the last round would fancy their chances of getting to the final.
'Ulster are favourites all right but they will also be nervous and apprehensive just as Toulouse were nervous and apprehensive before the quarter-final at Murrayfield. There's something about Edinburgh and the Heineken Cup which makes anything possible.
'Leinster have been absolutely brilliant in the way they have managed their senior players and brought the best of their academy players through. Their player-welfare is first-class and that all helps make them the ultimate professional team.
'You cannot fail but admire them for their level of performance week in, week out. Having said that, it will be really tough for them in Bordeaux. Leinster play a super brand of rugby but so do Clermont which is what makes it a difficult tie to predict with any degree of certainty.'
Another Scottish team is on course to be heading for Dublin next month. The Warriors of Glasgow need a home win over Connacht during the last round of PRO12 matches on May 5 to finish fourth in which even they will meet Leinster in Dublin in one semi-final with the likelihood of Ospreys colliding with Munster at the Liberty Stadium in the other.
As the season draws towards the climax of the Grand Final at the end of next month, Irvine describes it as 'hugely successful. Overall attendances are up by 5.3 per cent with big increases in Scotland, admittedly from a low base, and in Ireland.'
Leinster have increased their home crowd for the PRO12 from 17,000 to 19,000, a level which will put them comfortably into the top ten best-supported teams across the three major European Leagues. They are not the only Irish province to raise their popularity over the course of the campaign.
Connacht, flying the flag on the west coast in stirring defiance of their historical status as the smallest of the Irish provinces, have more than doubled their average attendance, from 2,269 last season to 4,653. The figures for both Scotland's representatives show a significant improvement on last season's figures of fractionally below 3,000.
Edinburgh are up to almost 4,500, the Warriors to nearly 4,000 with the prospect of a bumper crowd still to come at Firhill for their last match of the regular season. And if the next one after that proves to be a semi-final against Leinster in Dublin, then the defending European champions will not need to be reminded of who beat them before they set off on that run of one defeat in 25 - the Warriors at the RDS on September 17.
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