Sorry it's a bit 'old hat'; you must have been dealing with GTA SA questions for 4 years now...
I promised myself a PS3 only when I'd fully completed all my existing games on PS2. I'm on 99.47% completion with GTA SA and can't seem to finish the last 0.53%. I've gone through the checklist and there's nothing missing... I've got gold in all the schools, I've done all the missions, got all the properties, found all the tags, oysters, photo ops and horseshoes, and done all the Odd Jobs...etc.
The only thing that I can see as being an issue is that since the end of the San Fiero missions, a yellow 'destination' marker has stayed on constantly (it doesn't flash) on the garage property at Doherty. When I go there, there's the spinning 'save' disk, but nothing else (except the spinning weapons, of course). There's absolutely nothing that I can interact with to get rid of it. If I go back to my earliest saved games (41% completion), it's still there.
Is this a glitch that indicates that I haven't completed something, even though the game has allowed me to carry on? Is there anything I can do about this without starting again (which I won't do- therefore will just have to accept it)?