Note: For all bugs, even confirmed and "not a bug", it can be beneficial to the developers to post your experiences in either this thread or the Unofficial Patch 1.2 Thread (found here: http://www.gamesas.com/index.php?/topic/1300359-unofficial-patch-12-thread-6/). Just don't make a new thread unless it's a totally new bug.
Confirmed -- Either through extensive testing or from independent confirmation from a bunch of sources.
Unconfirmed -- Conflicting reports, etc.
Not A Bug -- Not really the fault of the 1.2 Patch.
Confirmed
- Elemental and Magical Resistances no longer work. The SKSE plugin has an unofficial, unsupported fix for this. You can find SKSE here: http://www.gamesas.com/index.php?/topic/1302434-wipz-skyrim-script-extender-skse/ , and a method of testing here: http://www.gamesas.com/index.php?/topic/1299511-testing-dragons-and-elemental-resistances-for-pc-users-on-patch-12-need-unpatched-people/
- Bookcases can forever eat books, especially ones in your home. (Not yet confirmed if this only happens with existing bookcases.)
- Dragons can start flying backwards and generally be unkillable in this state. It seems to occur only with dragons that are scripted to spawn (quest dragons), as self-spawned dragons don't do this.
- Treasure Chests (from Treasure Maps) are no longer spawning in existing games, and/or have vanished. Note that some chests don't spawn until you get the associated treasure map!
- Multiple problems with controllers and keymaps. There's an unofficial, unsupported quickfix by replacing controlmap.txt here: http://www.gamesas.com/index.php?/topic/1298537-having-trouble-with-your-360-gamepadfix/page__st__20__p__19561742#entry19561742
Unconfirmed
- Reports of textures being a lower resolution than in 1.1 on the same settings.
Not A Bug
- Disappearing main menu is almost always caused by third-party mods incompatible with Patch 1.2. Vals Smithing / Fletching Mod infamously causes this in Patch 1.2. It's a good idea to always disable all mods if you have trouble with a new patch, just so you can see if that causes it. There's an updated version of Vals Smithing / Fletching Mod on Skyrim Nexus that may have a fix. Remember, we don't have the CK yet, so some of mod-making has to be guesswork.