When you say "he doesn't fail to disappoint", to me it has a negative meaning as in he always disappoints. But I've heard some people using it as a compliment and in a positive way. So, am I wrong about the meaning or it's the other way around?
For example, Christina said on Voice:
My glam team doesn't fail to disappoint and they took the Monday blues and turned it into a stunning look any girl would love.
"Never fails to disappoint" in the OP's quote seems to be a good example of what the linguists on Language Log call a misnegation, whereby the utterance means the opposite of what was intended. Here is a sample of the misnegations they analyse:
The Republican Party is now at record low levels of unpopularity.
Boehner and Reid are in an undeclared war and neither is refusing to budge an inch.
There is no limit to what he might not ask.
There are many more of these at: http://languagelog.ldc.upenn.edu/nll/index.php?s=misnegation
I wouldn't say it means that they always fail, but that they always disappoint — i.e. they're reliably bad :)
For example, from The Observer:
"10 Great Whiskies" (December Observer Food Monthly) became slightly befuddled when it recommended the single malt Highland Park as "a genuine classic which never fails to disappoint". We meant, of course, "a genuine classic which never disappoints". Apologies.
Generally, I'd say that "never fails to disappoint", with an 's', is more common usage. Personally haven't heard it used in the way you describe above.
Possibly, it's a mistake by the user, or the same syndrome that's causing people to say "I could care less", when they mean the exact opposite.
If you use the idiomatic phrase "never fail(s) to disappoint" it only has one meaning. This means that the person whom the idiomatic phrase is addressed to will not live up to expectations, whatever that might be. If people are using it with a positive connotation (meaning the subject will live up to and possibly exceed expectations) that is 100% incorrect.
Never, fail to, and disappoint are all negatives.
That's three negatives, and the classical rule would be Triplex Negatio Negat.
However, there's a whole lot more to it than that, as Larry Horn's paper demonstrates.
Only the sort of people who often receive this "compliment" often can possibly think that it is one.
"Never to fail" is positive.
"Never to fail" to do something negative, like disappoint, isn't positive.
The whole sentence contains a triple negative: never + fail + dis, which amount to a negative.
It is a veiled insult: an ironic form of insult delivery which is misinterpreted as flattery to the buffoon who is targeted by it, much to the entertainment of anyone else within earshot who understands the true meaning.
The sentence:
My glam team doesn't fail to disappoint and they took the Monday blues and turned it into a stunning look any girl would love.
Is basically incorrect, not to mention a clumsy use of a coordinating conjunction. You want something like the following, with a different word in place of "disappoint", like "deliver":
My glam team, who never fail to deliver, took the Monday blues and turned into a stunning look any girl would love.
Christina is clearly not the brightest bulb in the chandelier.
"He doesn't fail to disappoint" or "He never fails to disappoint" simply means "He always disappoints".
