In writing, this is known as a "reversal of expectation" — that is the actual term used.
If you're looking for a single word, this comes close:
(esp. in a work of literature) an effect of anticlimax created by an unintentional lapse in mood from the sublime to the trivial or ridiculous.
In fact, Simon Armitage uses "reversal of expectation" in his definition of bathos:
This literary term means anti-climax. Julius Caesar declared, "I came, I saw, I conquered." That sequence of sentences is building up importance, and reaches a climactic point. Imagine if you were to arrive at school and say, "I came, I saw, ... I went home, I was just too tired!" This would be an example of bathos. You have let the reader or listener think that you are building up to a climax, but actually, you are deflating the statement, with a reversal of expectation at the end.
Although Armitage uses the term to describe an anti-climax in a series, there is no reason you cannot use the technique as a one-off. Given any plain declarative statement, the expectation of the audience is that what is forthcoming will be at least a nominal description; subverting that expectation toward a disparaging or self-deprecating observation may come as a funny surprise. Consider the following exchange:
Q. We're cutting corners to meet a deadline, but what will we do about the code review?
A. We'll fall off that bridge when we come to it.
The normal answer here would have been "We'll cross that bridge when we come to it," meaning we need to solve one problem before dealing with another. The disparaging twist that subverts the cliché says much the same thing but uses bathos as a humorous way to defuse anxiety.
The use of ironic understatement should not go unmentioned here. Probably the most famous example of this was given by (who else?) Mark Twain, when he countered the newspaper story which said he had died. He wrote:
The report of my death was an exaggeration.
At face value, this statement debunks the story outright, subtly mocking the penchant of journalists to overstate their stories (a practice that continues to this day), but also does much more. It suggests as well that Twain was indeed old and may be close to death, that his writing career was not as prolific or successful as it once had been, that his monetary fortunes had suffered a reversal (as they had), and so on. This type of ironic understatement is also called meiosis (which @RegDwight will be amused to know is another term for litotes), which seeks to gain a humorous effect by making things seem less than they ought to be.
Finally, this leads us to self-mockery and self-deprecation as candidates that fit your example (if you use your example against someone else it would simply be mockery or disparagement). Successful practitioners of such comedy, like Twain and, say, Louis CK, use the technique to get the audience on their side so that when they mock others they don't seem like bullies.