Nervous start for title holders
1999 Champions: Ulster
Opening game: Bourgoin 26 Ulster 12
After trumping the French throughout their inspired cup-winning campaign the previous season, Ulster's crown began to slip immediately when they lost 26-12 at Bourgoin. A dodgy scrum and a ponderous midfield undermined their challenge.
What happened next? Ulster failed to qualify from their pool, which became a familiar trend until 2010-11, when they finally reached the quarter-finals again after a 12-year wait.
2006 Champions: Munster
Opening game: Leicester 19 Munster 21
No jitters here, albeit a nerve-jangling denouement as Ronan O'Gara backed up his earlier dismissal of Sky's hyping of the English game by booting his side to a famous victory. His 80th-minute penalty in the fog, from inside his own half, ended the former back-to-back champions' 26-game winning streak at home.
What happened next? Munster qualified for the knockout stages but, after Leicester gained their revenge by becoming the first team to win in Heineken Cup fare at Thomond in round six, they failed to secure a home quarter-final and were shocked by Llanelli Scarlets in the quarter-final.
2008 Champions: Munster
Opening game: Munster 19 Montauban 17
Munster were nearly shocked by the competition debutants -- they trailed in the 74th minute after a Peter Stringer (right) penalty offence, but his long-time half-back partner O'Gara again rescued his side with a late penalty.
What happened next? Munster recovered their poise and after dismantling Ospreys in the quarter-final, they were short favourites to compile back-to-back triumphs before being stunned by Leinster at Croke Park.
2009 Champions: Leinster
Opening game: Leinster 9 London Irish 12
Leinster's last home defeat in the Heineken Cup was completed by a late Ryan Lamb penalty as Eoin Reddan kicked away a late penalty advantage that might have earned his side a draw.
What happened next? Leinster recovered to secure a home quarter-final, where they clung on to defeat Clermont, before crucial injuries conspired to see them lose an away semi-final to eventual champions Toulouse.
2011 Champions: Leinster
Opening game: Montpellier 16 Leinster 16
Another match-up with Heineken Cup debutants and another escape act, this time courtesy of Jonathan Sexton, who kicked a late penalty to maintain his side's impressive unbeaten European record.
What happened next? Leinster recovered to find their form and become the only Irish team to win back-to-back European titles the following May, when they hammered Ulster in the final.
2012 Champions: Leinster
Opening game: Leinster 9 Exeter 6
Leinster's difficulty with plucky debutants continued, but this time they managed to emerge with a hard-fought victory, despite walking a tightrope when the visitors missed a long-range penalty to level with the last play.
What happened next? Leinster will hope to demonstrate similar powers of recovery as evidenced in previous years in their bid for an unprecedented hat-trick of European titles.