So I looked around but wasn't able to find this anywhere, but the other day I found a nice little glitch.
If you're outside of a door that says "...barred from the other side.." no need to worry. First, stand in front of the door and have the "open door" command display, then go into your menu and exit back out. At this point the command will say, very quickly, "activate door bar". You will likely miss it the first few times since it happens so fast, but if you press the command button a few times you usually can get it.
I've tried it on at least 4 different doors, and was in 3rd person mode. Thought I would share this with you, unless you already knew of course.
It works on all doors as far as I can tell, you can also do it by moving the cursor around in the area where the bar would be on the other side. Every now and then you will see the unbar door action pop up and disappear. Then fiddle around in that area while tapping action and you'll get the door open eventually. I don't do it very often though, mostof the locations are nicely designed to loop and I don't want to come up against a cliff that I should be jumping down from. It does help a bit on that quest wherer you have to get the falmer translation from the back of the dwemer museum though.
Yeah but Skyrim is quite forgiving in terms of being where you're not supposed to be. The older games less so. I've noticed that in most locations there is a lever or some way out of the other side of a locked door, a lever that you could only use if you weren't meant to be there. I guess they figured it was likely to go wrong sometimes so...
On the other hand, If you're just grinding quests (IE: to get Thane in Riften) Then it can be useful to pop in through the screen door in the back and snipe the bandit leader / target without having to face 20-30 minions in a 20 minute dungeon romp.