Its the first time i will talk about that, but it happened me long time ago, and each time i do it, it happen again.
Only the unoficial patch installed (the 3 patch, for oblivion, SI and the official pluggin)

The first quest is given after we murder someone. We have to go to an Inn betwen imperial city and the city at complete south of the map.
Everything goes well, i kill the guy, the brotherhood guy come see me after... then after my guy wash his hand, look at the beatiful corpse with a lot of blood, a kill maybe a little to much easy, since the NPC had only 1 life... Then, i go to the ladder to go up, i activate the ladder, and NOTHING. I the ladder dont activate, im trapped in the basemant. So each time i have to use the console to get out.

I dont like using the console, its like cheating. Anyway, i did not even remember what to write.

This happen each time i install oblivion. So its not just a corrupted installation. I installed oblivion on 3 different computer, using 2 different oblivion to try it. My oblivion GOTY and tried with my friend's GOTY to be sure it was not my disc that had a corrupted file.

Also, check that: http://www.uesp.net/wiki/Oblivion_talk:Inn_of_Ill_Omen

Hmm, I haven't encountered this bug in the 20+ times that I've played through the DB questline. Then again, I don't have the GoTY edition on PC. Maybe they mucked something up there?

I've run into this problem. The trouble is the activator for that ladder/trapdoor isn't in the expected place (IE, on the trapdoor or the ladder itself). You sort of have to pixel hunt for it. IIRC it's down and slightly to the left of the ladder. Just pan your mouse around until you find the little door icon.

If you really can't find it, since you're on the PC you can console your way out...just COC out to Bravil, the upstairs of the Inn, the Faregyl Inn, or wherever you want to be.

I've been through the DB quest who knows how many times and never ran into this, but if it's enough of a problem that people are having trouble getting out I can see if there's something the UOP did to aggravate it since that seems to be a common theme here.

I've been through this quest with the UOP loaded countless times too. I have to scan around a little bit for the door icon, but it's not too bad. It's usually on the upper part of the ladder, not on the trap door itself, which always catches me for a minute You'd think I'd know by now! Haven't been back to this quest in a bit though, so if the UOP did something, it would have to be in the last two versions of the supplementals.

Nope. UOP didn't do anything in that cell other than mark a bed as owned. It does look like the ladder is partially buried behind the wall's collision though so only the rounded front parts can be touched. I can pull the ladder out some to get more of it past the wall collision but if you're already in the cell that change won't take since doors are persistent references.

the first time i played oblivion with the unoficial patch was maybe 2 year ago. I had that glitch.

Maybe its the cell itself that its bugged, so when we modify the cell, the door bug. Maybe it just the Main .esm that was wrongly compiled. There is anyway to anolyse if a cell has error? When i talk about error, its not because something is not well placed, but more like if there was corrupted byte.

Nope. That's not the issue. It's definitely the collision. The basemant set used there isn't the greatest for collision. It's actually sticking out away from the wall itself, which interferes with the ability to click on the ladder. There's no corruption, the UOP didn't cause it, it's been like this since who knows when. It's just that not everyone noticed because we all apparently try to click on different parts of the ladder. I usually go for the rounded ends so that's why I never noticed it.

Yeah, I just wave my cursor around the ladder until the door icon shows up. I usually point it at the trap door, curse because then I remember I need to point at the ladder, then point at the ladder, get the icon and go. You may need to move your cursor around a bit until you get the door icon, but it will pop up eventually...