Another method (much more common for software that asks for two pieces of information, like a name and a key) is to take a memdump of the process at the "your key is invalid" dialog, find the invalid key you just typed, and hope that a valid key is somewhere nearby in memory. Unlike the assembly trick, this requires 0 programming expertise beyond the ability to type `strings` on the command line.

This works because some programs use a hashing algorithm to calculate the key based on the name, do a strcmp, and pop a messagebox if the keys don't match, without zeroizing the valid key buffer first. If the key buffers are on the stack (or if the two mallocs just happen to use the same region in memory), it is often easy to find a valid key if you know where the invalid one is.

I guess software that derives keys this way is far less common than it once was, but I know of somebody who cracked something using this method just a few years ago, so it still pops up from time to time.