After I'd posted "Salami, boiled frogs and broken windows.", I remembered there's also another name for this phenomenon that's a trifle less complimentary. It's the Slippery Slope Fallacy, and you learn about it in first-year Philosophy. It's also known as the Fallacy of the Beard, because the fallacy can be seen in the following examples:

If you pull a single hair out of somebody's beard, it's not going to make any difference at all. Therefore, since any individual hair-pulling isn't making a difference, you can keep doing it forever.

Or...

The act of pulling out a single hair could be the difference between having a beard, or not having a beard. Since we can't define the exact point at which the change occurs, we must say that any hair-pulling will destroy the beard.

We mustn't lose sight of the fact that the single slice of salami isn't thick, the slight change in temperature won't in itself kill the frog.