RC Toulon, Saracens, Stade Rochelais, Racing 92, Ulster and Munster are amongst those who could potentially crash out of the biggest competition in club rugby before the knockout stage in what would be a seismic shock to the tournament.
For Saracens, it could be the next chapter in a sorry season as their squad ages and their stars look to pastures new; the defending Gallagher Premiership champions sit a lowly sixth domestically and must win in London (1/12) against Lyon (12/1) to progress out of Pool 1.
Home advantage may be their saving grace but victory alone may not be enough – they have to match Bristol Bears and outscore them by at least two points to squeak through.
Lyon have looked solid and have already secured their spot in the next stage but expect them to lose here – if narrowly.
Bristol Bears travel to play Connacht in Ireland, a completely different task to facing the Irish province anywhere else. Bristol (6/4) have not picked up a point since their opening day bonus point win over Lyon, and losing at the Sportsground will likely knock them out. For winless Connacht (33/50) to progress they would need to win with a bonus and Sarries would need to lose - Connacht should fulfill their end of the bargain, but Sarries will surely scupper their hopes.
To Pool 3 and RC Toulon, who entered the tournament with 'grandes idées' but find themselves without a win. They have had a difficult time of it, with Exeter Chiefs and Munster Rugby both coming from behind to win in France while Northampton Saints struck late in England.
The French side (11/2) travel to Glasgow Warriors (4/11), who were denied victory at Exeter Chiefs only by the TMO, for what is effectively a qualification shoot out. RC Toulon are the better side, but Glasgow Warriors have shown a grit this tournament that the French club have not. With home advantage, a Warriors win by less than 5 is on the cards.
Elsewhere in that pool, Northampton (4/1), assured of a home Round of 16 tie, travel to Munster (3/10) who will secure their spot with a single point.
With Exeter (8/9) in France to face bottom side Bayonne (6/5), smashed by Northampton Saints last week, Munster's hopes of a home tie are slim, but this could be a high scoring affair even if pride is the predominant aim – Northampton are flying, and Munster looked reinvented against Toulon; expect points.
Pool 2 offers a thrilling three-way battle for one spot; only one of Ulster, Racing 92 and Cardiff will make it through, all of whom have shown flashes of brilliance but considerable inconsistency.
Racing (1/33) are in the best position in playing Cardiff (40/1) at home - the Welsh region have stuck firm to playing rugby their way and that has resulted in two bonus points, but no more. Racing are similar, with three bonus points, but look more like the finished article.
They should run in a big score, but it will all depend on Ulster (11/4) in London against Quins (6/17) - a win of any kind for the Ulstermen will be hard to come by at a buoyant Stoop. Expect a solid win for the hosts by around 10, and for Racing to progress (and cause problems for the higher seeds in the knockout stage).
Finally to Pool 4, where La Rochelle (10/21) travel to face Sale Sharks (23/10) in what is effectively a knockout match; the champions sit on seven after picking up two losing bonus points and a bonus point win against Leicester Tigers, while Sale Sharks are two behind them.
That means Sale Sharks could win (without a bonus points) and still go out to the holders if the French side get two bonus points. That's an extraordinary scenario – but it's likely Sale will be on their way regardless.
La Rochelle will dig in, kick points and hold firm defensively to sneak their way through after a poor start to the tournament, but those first two losses could come back to haunt them as they will likely be without home advantage in the knockouts.
Their odds at winning the tournament for a third straight time are considerably longer than they were a few months ago at 8/1. Leinster Rugby remain favourites at 2/1 and Bordeaux, who have been extremely impressive, have leapfrogged them to a 7/1 price.
But it's Toulouse at 3/1 who have been the most impressive team so far. Their odds will only get shorter as they march towards the final, and while Antoine Dupont will be with the Sevens side for the Olympics, they have a slew of backs ready to take his place.
Another Leinster-French match-up could well await in May's final.
