Okay. Looking things over here's what I see are the specifics.
Mianly from page 85 of the PHB:
First. The UMD Character must have *deciphered* the scroll, and simply not be told what's on there. Else he needs to make a *decipher a written spell* (DC 25+SL)or *spellcraft* (DC 20+SL) check. This takes a minute but can be done ahead of time.
Second. The UMD CHaracter must have the Ability requirements to cast a spell. If not, he must also make a *Emulate an ability score* as per the description of the UMD skill on p85.
Then you can make a UMD roll (DC 20+Caster Level) to cast the spell.
Now, in the DMG (p238) if a character who can normally use a scroll (someone not UMD'ng) meets the Ability, has the spell on his list and is of equal level as the scroll's creator, they automatically do it. (all things simulated by the uMD skill check.)
If the "legit" scroll caster does not have the level equivalency they must make a CL roll (DC scroll's CL+1), failing that requires a WIS check, DC5, not to have a mishap.
The way it reads...
UMD allows you to ignore the "must have it on your spell list" requirement. And the DC 20+CL for the UMD roll takes care of the "caster level check".
But if the UMD user fails the "caster level check/use scroll roll" there should be a penalty...as there would be for a wizard who studied and used magic all ther "life".
So I would say that a WIS (DC5), check would be in order for a fail'd UMD roll of casting a spell. (This is what a Wiz or cleric who failed the CL check would need to make.)
I mean...its a DC5 on a WISDOM check...who wouldn't make that like 99% of the time anyways...
