There are many cringe-worthy moments (not ranked):
1) Early Years: Bono's voice - half-punk/half-singer = bad singer. Sounds nasally and he's forced to scream.
2) Early Years: Repeated songs
3) Pride: Every version of this song ever performed by U2 is not as good as the album version. They captured something magic in the studio and have been unable to reproduce it in concert. This is true whether it was 1984 or 2005
4) With or Without You - The 90's and Beyond: U2 made this beautiful album song soar in concert, frequently adding extra lyrics and kick-ass guitars. Bono's voice truly sailed on this song. Then came ZOO TV. And what was once a magical moment in concert became an eye-sore. Their more abrupt version of the song failed to do anything. Maybe this was intentional as U2 try to shake of the shackles of JT, but I think it did a disservice to the song and the fans. U2 have redeemed themselves a bit, especially on the Elevation Tour, by adding to this song again, but it never has regained what it was in the late 80's.
4) Las Vegas - PopMart Opening Night: Blech! "Do You Feel Loved" was horribly out of tune and the band was not prepared.
5) Early part of UF Tour: Supposedly the band took a mini-break to practice their songs again as even they admitted to not being good at the start of the tour.
6) First Night Chicago - Vertigo Tour: Again, band was just off. Vegas and the start of the UF tour could be forgiven as this was the first time U2 were playing these songs in concert. But by the time they hit Chicago on the Vertigo Tour, they had many play-dates done. U2 were just bad and the band admitted it.
7) Complain all you want about Bono's preaching now, but it pales in comparison to his JT era rants. Some of those were truly cringe-worthy. At least now, there is an immediate cause and effect - Bono isn't just ranting, he's getting things done. Back then, it was juvenille.
8) The oft-mentioned Sydney show where Adam was too hungover to play: enough said.
9) LA - JT Tour: U2 tried something different at the end of the concert - they sat on stage and tried to have a chat and interact with the fans. Spectacular failure!
10) Repeat of "Vertigo": While one might forgive early U2 for repeating songs as they had a limited catalogue then, there is ZERO excuse for repeating "Vertigo". Yes, it's a great rocking song. Either launch the night that way or end it that way - but repeating it, unless it's like the 25th song of the night in a third encore, is pathetic at this point in U2's careers. They have SO many other songs fans would love to here live.
11) PopMart Tour: The ZOO TV tour was fun. It was still held in arenas (in the U.S. at least) and combined flash and intimacy. Even in stadiums, it worked. But PopMart was over the top and the songs lost a lot of meaning. The only high points were on the b-stage (like the acoustic "Staring at the Sun") as U2 removed themselves from the excesses of their stage design.