His biggest mistake was his carrying on in Frank Pentangeli's living room about the abortive hit. It was pure theatrics to fool whomever might be listening, and perhaps to make Pentangeli nervous as well. Of course thereafter Mike confides in Pentangeli that he knows it is Roth who is out to get him. It is also in this scene where Mike tells Pentangeli to keep his friends close and his enemies closer. I think this left a doubt in Pentangeli's mind about where Michael was coming from and it led to the whole COngressional investigation.