By committing to attend Congressman Paul's press conference only to declare minutes before hand that "it just isn't worth it." This is according to Don Rasmussen, Events Coordinator of the Campaign for Liberty.
Bob Barr has been slamming Dr. Paul publicly since he won the formerly-libertarian party nomination, but this seems to have offended more Ron Paul supporters than anything he has done so far.
For those who don't know, Barr spent much of his life jailing people on behalf of the federal government for possessing substances that the state disapproves of, before recently "seeing the light". Even since his supposed revelation, he has praised the troop surge, argued for intervention in Iran and South America, advocated a national sales tax, and voted for the PATRIOT Act twice. This just scratches the surface, of course.
With that said, it's probably for the best that he didn't show up today.