There was a certain person who was called up as a witness at a wedding. One of the people there told the officiating Rav that this man is a thief and pasul li-eidus.
But the Rav didn't believe him. He is but one person and has no ne'emanus to say that this man is a thief.
So the "show went on" and the chosson was mekadesh the kallah with this putative thief as witness. Then they called his accuser up to say a bracha. The Rav told him not to make a bracha because in his mind the kiddushin weren't valid.
Was he correct??
Well, firstly, if the Rav is correct, he should not say amen to all of the other brachos because in his mind all of the brachos are in vain.
It could be though that even if the person WAS a thief, he still may act as a witness. The Chavos Yair [Hashmatos P. 255] discusses one who witnesses an event and wants to testify but knows that he is pasul li-eidus while the Beis Din has no inkling of this, may he go testify anyway?
The Toomim [87/27 and the Chasam Sofer Yo"d 11] says that it depends why he is pasul: If he is pasul because he is a blood relative then he may not testify because there his disqualification is absolute, a decree of the King. There is no way around that. However, if he is pasul because he is biased [נוגע] or a rasha, then he may testify. For in this case the concern is that he might lie - חשש משקר - and he KNOWS that he is not lying. [However the Nesivos - the Litvisher not the Chasidisher:-) - says that נוגע is pasul because he is like a baal davar and relative while a rasha is pasul because חשש משקר].
However, the Yeshuos Yisroel  and the Chiddushei Harim argue that he may not testify in any event. For besides the חשש משקר of a rasha and נוגע there is also an inherent פסול הגוף.
The Chiddushei Harim kindly provides proof for his assertion. The gemara [Shvuos 30b] says that if one knows that his friend is a thief he should not join up with him and testify, as the pasuk says מדבר שקר תרחק. So too, if he knows about HIMSELF that he is pasul, he may not testify.
The Ktzos [ or "Ktzois":-) 28/8] also says that if one knows that he is פסול לעדות he may not testify. However, as long as he does teshuva in his heart, he can once again testify and it is not necessary to actually break his gambling tools [if he is a gambler] as one must do if the Beis Din knows that he is פסול לעדות [Sanhedrin 25b].
If one says that a shochet is pasul li-shchita, the Shach [Yo"d 1/41] says that this person may eat from this shochet's meat because even though שויא אנפשיה חתיכה דאיסורא - He is saying that it is assur for him, nevertheless, maybe the shochet did teshuva. So too, in our case, let us assume that this thief did teshuva. However many poskim [Pri Chodosh 1/42, Pri Megadim 304/41, Gra 1/43] take issue with the Shach and don't assume that he did teshuva.
There is an interesting nafka minah: If this woman who was married was subsequently raped and gave birth to a girl, we assume that this child is a mamzeres. But as far as the man who said that the witness was a thief is concerned, the child is kosher because her mother was never married. He can therefore marry her himself. And if HE is a mamzer he may NOT marry her because he claims that she is pure. But if he rapes her then he must fulfill Torah law and marry her [if she wishes] because his שויא אנפשיה חתיכא דאיסורא doesn't have the power to uproot the mitzva of marrying his אנוסה [or any other mitzva for that matter] .
The untershte shurah - bottom line - is that the Rav was correct and this man should not make one of the sheva brachos after claiming that the witness was pasul.
[מאוצרות הגר"א גנחובסקי זצ"ל]
As an interesting aside - Rav H. Schachter Shlita relates that he was once present at a wedding where a Rabbi was called up to be a witness but he declined the honor. A few days later the story came out in the papers that had stolen and was caught. Well, at least he was honest enough not to mess up the kiddushin with his עדות פסולה....
Based on what we learned from the Ktzos it would have been enough for him to do teshuva for his dastardly deeds and he would have been instantaneuosly kosher. And maybe according to the Toomim and Chasam Sofer since there is no חשש משקר he would be kosher since nobody knew. ויש להאריך.