Bolty's Note: out of the blue, this post truly rocked the DSF, surprising everyone.  It led to a massive thread discussing the implications and legitimacy of using this item bug.  It surprised Blizzard, too.  Here's the short and sweet post:

The "of thieves" suffix, which says it "absorbs half trap damage" doesn't describe itself properly.

In fact it absorbs half of trap damage, monster spell damage, and monster arrow damage.

Read that last sentence again, if you're not already using "of thieves".

The damage reduction order, in case you're curious, is:
1) If you have the "of thieves" effect, cut damage in half
2) +/- any absorption or +damage effects [e.g. stormshield; scavenger's carapace]
3) if the damage is now below 1, make it 1
4) if it's magic/lightning/fire damage, reduce it by the % of your resistance to it
math note: damage [and life and mana] has 6 hidden bits of precision, so fractional damage is possible.

The above information does not apply to attacks from a player to another, however indirect [or even cases, like firewall, where you attack yourself].

BTW, Empyrean Band, a single player only quest reward ring also has this "of thieves ability", but is the only unique item to do so.

-- Crys

p.s. I consider it amazing that, playing Diablo since January 11, 1997, I only learned this yesterday. I do not believe it is well known.

Bolty's Note: in fact, it was only known by 2 or 3 other people Crystalion had discussed with privately about the bug.  The bug was fixed months later in version 1.07 of Diablo.  Like P(IG)'s Cudgel, the suffix "of Thieves" was referred to by many DSF'ers from then on as "of Crystalion" to honor his discovery.

Looking at Jarulf's new v1.10 guide to Diablo/HF I see the "value" of "of thieves" is medium low [11], which means that you should not find it from crummy monsters, but probably not from tough hell monsters either. Your character will probably be at least a teenager before Gris starts offering it.