I was certain that I posted this before and had several replies, but darned if I could find it by searching and looking at my posting history.

Anyhoo...a friend of mine, who I don't think posts/lurks here, went to the Beach Boys' concert in New Haven, Connecticut on May 15, 1965. He remembers 1) being cheesed that Brian wasn't in the lineup, and 2) that Glen Campbell was given a solo spot during the show and performed one song, a song my friend had never heard, at least not by the Beach Boys. He doesn't remember the song at all, but over the years after things he learned about Brian and such, he thinks it may have been "Guess I'm Dumb." And he says he's sticking with that unless someone can definitively say it was something else.
I have two thoughts....1) I doubt that the Beach Boys at that time could have pulled off "Guess I'm Dumb" as a band, but 2) he could have sung to a backing track or lip-synched to the track or have had another set of musicians backing him up, and 3) given that "Guess I'm Dumb" was released as a single on June 7 of that year, just a few weeks after that concert, it would be plausible that he would have been promoting the song.
Can anybody confirm or definitively contradict that Glen Campbell sang "Guess I'm Dumb" in concert when he was a touring Beach Boy? The concert isn't documented on Eric Aniversario's setlist page, so that didn't help...