The Blues back row wizard pushed his kick from in front of the posts on the 22 wide to the left of the posts to allow Tigers No 8 Jordan Crane to step up and steal the glory - and a ticket to meet Leinster in the Heineken Cup final in the Scottish capital.
It was a sad end to a phenomenal match in which Williams had once again been one of the stars in a Blues side who scored two tries in the final five minutes of normal time to force a 26-26 draw.
That brought on 20 minutes of extra-time in which neither side was able to score again. In the end, the Millennium Stadium hosted the first penalty shoot-out in world rugby in a game of this stature.
Lions 2009 tour manager Gerald Davies didn't know whether to laugh or cry at the end of an incredible weekend of Heineken Cup action.
On the one hand there was the agony of seeing his former club fail to reach the final in the cruellest of ways, while on the other there was the joy of realising that the showpiece occasion won't now be littered with Lions.
Had Munster, who went down 25-6 to Leisnter in Dublin, and the Blues made the final then there could have been 13 Lions tourists battling for Europe's highest club rugby honour just a day before the Lions leave for South Africa.
Now it means there will be only three from Leinster - Brian O'Driscoll, Luke Fitzgerald and Jamie Heaslip - and one from Leicester - Harry Ellis.
The Tigers finally triumphed after 100 minutes of bone crunching rugby and then 14 place kicks. The Blues appeared to have the upper hand when Tigers wing Johne Murphy missed to leave their rivals needing only one score to clinch their ticket to the final.
But the pressure got to earlier try scorer Tom James and he missed to leave the game open at 4-3 to the Blues. Another try scorer, Tigers' Scott Hamilton, levelled with his kick and then Lions centre Tom Shanklin and Aaron Mauger swapped kicks.
At 5-5 the two skippers had to nominate another five kickers each, bringing the forwards into the equation. Ricky Rees kept the Blues ahead as the competition went into sudden death before former All Black flanker Craig Newby scored.
Next up was Williams, and his miss allowed Crane to win the game.
Penalty Shoot Out Sequence
Ben Blair scores 1-0 Blues; Julien Dupuy scores 1-1; Nicky Robinson scores 2-1 Blues; Sam Vesty scores 2-2; Leigh Halfpenny scores 3-2 Blues; Geordan Murphy scores 3-3; Ceri Sweeney scores 4-3 Blues; Johne Murphy misses, 4-3 Blues; Tom James misses 4-3 Blues; Scott Hamilton scores 4-4; Tom Shanklin scores 5-4 Blues; Aaron Mauger scores 5-5; Ricky Rees scores 6-5 Blues; Craig Newby scores 6-6; Martyn Williams misses 6-6; Jordan Crane scores 6-7 Leicester - Leicester go through to final.