On the subject of Dylan, saw him about 8 years, having seen him years before and been enthralled. he was ... er... not very good.

William Shatner popularised speaking rather than singing the words. Dylan extended this idea by shouting them, (especially when he forgot them.)

We could knock a few people off a few pedestals with this post, but perhaps an acknowledgement that the best have bad days even though we pay to see them at their peak each and every time? (As a football fan, I could wax lyrical about this dilemma.)

Acts I used to revere, (and still do) but must have been a bad day when i was there one time include;

Ewan McColl. (Poorly, but game enough to turn up. Peggy was busy that night.) (1985 ish)
Martin Carthy. (seemed to be going through the motions.) (1990 ish)
Dick Gaughan. (seemed to be orbiting a different planet that night.) (2003 ish)
Fairport Convention. (Ok, blame the sound engineer, but...) (1980 ish)
Bob Dylan - see above (2003 ish)
Pogues - (McGowan had possibly been lubricating his vocal chords ready.) (1984 ish)

In contrast, I have never seen a bad performance by some acts. Off the top of my head, they include Van Morrison, Maddy Prior, Vin Garbutt, Planxty, Chieftains, and many more.