Definately that 'no recip found' bullshit that grzepa just mentioned. That pisses me off enough to go outside and find someone to smack. I've heard someone say that that is due to an html error in my galleries. Bullshit. Bullshit. Bullshit.
I love sites that don't tell you that they only accept partner submits AFTER you fill out their form and submit. Why not post that with the form? Assholes.
Scripts that don't accept the words 'pose, poser, or posing' in the description. Oh wait, that's me.

(George gave me the idea!)
I've a huge problem with sites that say 'give your gallery a good description to make it stand out in the crowd' only to found out that there is a 17 character max on their descriptions. What can I describe in 17 characters? 'This TGP sucks' -and I still have 3 to spare.
Or...the fuckers that tell you they want a 90x120 thumb, but the script rejects it because it is now set to find a 90x90 thumb.
Or the brand new TGPs with even less traffic than mine that want some off-the-wall sized thumb that no one else has ever wanted. Christ!