They have very fine metal fingers deposited onto the saw material. So if you exceed the RF input voltage you can either vaporize the fingers, or screw up the contact metal-to-saw material.
That said, the data sheet rating is probably for long term exposure. If you accidentally over powered the input for a few seconds, it probably survived. If you left the power on continuously for a while, it might have degraded the fingers.
Some researchy type saws (like the type deliberately made for low phase noise oscillators) have beefier fingers to handle much more RF power (see Leeson's equation!).
Rich