Something I just thought of: anyone ever see someone train to failure on the angled leg press? That is, the person actually missing his last rep? I seriously can't recall ever seeing it.
Failure is such such a subjective term. One persons failure is where another could have squeezed out another 1-2 reps, even being of equal strength.

One person who always looked like he really squeezed everything out of his sets is Rich Gaspari, just from the short clips I've seen of him training. There's some new clips on MD where he trains and he looks like someone whose "failure" point is higher than most.
There can also be a 100% difference in how taxing the last possible rep on a set is, depending on how long you had to really strain for example. And of course, rope pushdowns to failure aren't very taxing whereas a deadlift 1-2 reps from failure can be a 1000% more taxing.
What matters is progress. Load and/or reps with same load. Nothing special happens at the failure point, unlike what Mentzer claimed.