I've always been under the impression that one can only be hit with Crushing Blows by mobs at least 3 levels higher.

http://evilempireguild.org/guides/crushing.php

http://www.wowwiki.com/Formulasefense

I did read that crushing blows by mobs less than 3 levels higher are possible if one's defense skill is less than one's levelx5. Ex. <300 at lvl60, <350 at lvl70.

http://evilempireguild.org/guides/crushing.php

http://www.wowwiki.com/Formulasefense

However, my defense is definitely maxed out at 350, and yet I was hit by lvl72 Hellfire Warders in Magtheridon's Lair with crushing blows. Another tank in my raid also reported being crushed by the Warders.

Did the Crushing Blow mechanics change, or am I on crack, or is my combat log mod bugged (I use EavesDrop)?

Any source?

The fact that patchwerk doesn't crush and is level 63/73? Same with Loatheb?

The fact that 71's and 72's can crush? The fact that crushings go up if you get UBW'ed and go below 300 defense back when we were 60? Plus i'm pretty sure this is well known.
You want a source? Me. I may be wrong, but searching through wow wiki will only give you the dialogue of C'thun's third phase. You can do the testing though if you like.

Edit: Ok, maybe a bit hostile, but still. You said it yourself. You saw the 72 crush you. Most mobs carry weapon skill based on their level, hence why MOST are +15 skill/% crush. There are exceptions however, and this can't be explained by "level."

The Evil Empire guide was very good, but it's out of date.

It also says you should never get glancing blows on same level mobs, which I have, and continue to see. (Feral weapon skill is automatically 5x level.)

I don't think this is correct, but I'm not certain.

Getting crushed by a +2 is unusual. I'm pretty sure we weren't getting crushed by +2's since the beginning of time. We saw back in Silithus that mobs can have boosted Defense; there's no reason to assume that they can't have boosted skill.

Also, there's no reason to assume that Patchwerk has 300 skill (unless people have crit-less parses after tanking him for extended periods of time with 425 Defense). He probably just had a "no crush" flag, which we've seen with Hydross that they can do at will.

Finally, crush chance scales unsually quickly if you're below min skill. I recall reading somewhere that Druids who tried to tank Vek'nilash would rocket past 100% crush after a UBS.

If it's an isolated incident then it's just the mobs.

If it's all 71-72 mobs then the mechanic has changed.

Which is it? The thread on the R&D boards was giving me a headache. I'm prone to believe the former rather then the latter on the grounds I've tanked most heroics and those 72 bosses at the end don't crush.

I do indeed with a 2.0.0 version of Combat Monitor was out. =/

Its very possible I'm wrong, I remember people also claiming Patchwerk was 310 weapon skill, but level alone doesn't mean anything to crushing, other than it tends to work out that way as 315 is 15% crushing for a 60.
Weapon skill should dictate it fully... its also possible there is a crushing blow toggle, but I've been crushed before on many trash mobs... I just don't recall which.

It may be that they design the trash with higher weapon skill, so that the normal hits are mitigated more by the tank's armor, but there is still potential for spike damage. This way healing can be distributed amongst the entire raid via AoE abilities, without driving the raid OOM.

This was brought up in the Magtheridon thread - as far as I know, the trash leading up to Magtheridon is the only sub73 mob that can crush.

The crush rate is pretty high - I'd estimate around 15%. I.e. the same rate as from a lvl 73.

Crushing blows AFAIK can be done by any higher level mob (or more likely mobs with weapon skill > your character level x 5) but are reduced by defense up to character level x 5, with the rate set such that with character level x 5 defense, a standard +2 mob (or +10 weapon skill compared to your lvl x 5) does 0% crushing and a +3 (or +15 weapon skill) does 15%.

This is backed up by the Unbalancing Strike parses on druid tanks. Druid tanks were reduced by Unbalancing Strike far below lvl x 5 defence, and the result that EVERY blow would crush. They couldn't even be crit, because crush filled the table. This proved that defence and crushing blows are related.

A similar argument was the high rate of glances against certain sub-63 mobs in the old days (e.g. elite Silithids), and whether that reflected mobs with a high +def or a "glancing blows" flag turned on. Actually, we could settle that one now and also work out how much +def it is (it's totally +def, y'all) by parsing the glancing rate of 60s, 61s, 62s, 63s and 70s vs those mobs.

You glance on normal mobs that are even +1 level, but only get crushed by vs mobs that are +3 or higher with "normal" level of Defense/Weapon Skill ("normal" being skill 5x mob level). I'm fairly certain increased levels of glancing blows (such as on some 70 mobs you can glance on, or on Gruul, who has a higher than normal glancing percentage) are based on the mob having elevated defense. And Patchwerk/Loatheb had -5 Weapon skill from a regular +3 mob, etc. This would lead to the idea that the trash in Magtheridon's lair has 365 weapon skill (5 more than normal).

It is more than obvious that different mobs are granted different melee and defensive stats. For example, there are mobs flagged as warrior or paladin types that have Deflection, so they have 10% parry instead of 5% (The Four Horsemen come to mind).