The rules on using magic items are not at all clear to me. Especially the Spell Completion and Spell Trigger activations are confusing. What kind of character (class, level) can use a wand (spell trigger)? What kind of character can use a scroll (spell completion)? How does the Use Magic Device skill work in all this?
Spell Trigger: Spell trigger activation is similar to spell completion, but it's even simpler. No gestures or spell finishing is needed, just a special knowledge of spellcasting that an appropriate character would know, and a single word that must be spoken. Spell trigger items can be used by anyone whose class can cast the corresponding spell. This is the case even for a character who can't actually cast spells, such as a 3rd-level paladin. The user must still determine what spell is stored in the item before she can activate it. Activating a spell trigger item is a standard action and does not provoke attacks of opportunity.
My players' characters pried a Wand of Stoneskin (a 4th-level wizard spell) from a dead wizard's hands. The rules of spell trigger say that the item "can be used by anyone whose class can cast the corresponding spell [...]". Does that mean that a 1st-level wizard could use the wand, even though he could only cast the spell at level 7 (being able to cast 4th level spells)?
Spell Completion: This is the activation method for scrolls. A scroll is a spell that is mostly finished. The preparation is done for the caster, so no preparation time is needed beforehand as with normal spellcasting. All that's left to do is perform the finishing parts of the spellcasting (the final gestures, words, and so on). To use a spell completion item safely, a character must be of high enough level in the right class to cast the spell already. If he can't already cast the spell, there's a chance he'll make a mistake. Activating a spell completion item is a standard action (or the spell's casting time, whichever is longer) and provokes attacks of opportunity exactly as casting a spell does.
How about a Scroll of Stoneskin? The Spell Completion description speaks about "a chance he'll make a mistake" (if the character is not of high enough level in the right class). Does this mean that a 1st level wizard, or even a 20th level fighter, could read and use the scroll (accepting the chance of failure)? Does this involve the Use Magic Device skill? What's this mentioned chance of failure, and what can be the mistake?