Pettitte testified on Wednesday, and while under cross-examination, said that there was a "50-50" chance that Pettitte misunderstood the conversation between he and Clemens in 1999-2000 regarding Clemens' human growth hormone usage. Needless to say, a 50-50 chance not only fails to diminish reasonable doubt in a courtroom, it creates reasonable doubt in its own right.
Pettitte has been the government's key witness as they try to prove that Clemens is guilty of perjury after allegedly lying to Congress regarding his usage of PEDs.
With Pettitte's testimony dealing a big blow to the government's case against Clemens, it's likely that Clemens will be acquitted of perjury. Federal prosecutors will now have to rely heavily on the testimony of Brian McNamee, who is a shady character to say the least.
Clemens' legal team has already filed a motion to show how poor of a character McNamee is. Needless to say, this motion is now even more important after Pettitte's testimony.
McNamee's testimony alone likely won't be enough to prove that Clemens lied to Congress, especially if the judge grants Clemens' motion. With Pettitte's "50-50" statement, it looks like Clemens will almost certainly be acquitted of perjury, as there simply won't be enough evidence to convict him.