I installed WavTap via the installer, running OS X 10.7.4.
I can start recording, but the application crashes once I try to save. I restarted it several times via Spotlight, always with the same outcome.
No files were recorded (unless the default folder is something other than the ~/Desktop).
Can you paste a Crash Log here?
Also, for clarification: this happens only after clicking "Save the last 20 secs" and not after clicking "Start Recording", right?