When the framerate drops to something rediculous like 3fps for about 1-2 seconds. Can happen any time and is hard to reproduce. CPU is probably doing something intensive in the background and can't render the UI animations fast enough.
I get them on stock ROMs (w/ netarchy-toastmod kernels). less so when I run ADW launcher over the Sense launcher. I've always felt they were Sense-related because of this.
Nonexistent on CM7 and MIUI.
Hmmm. What sort of UI animations might these be noticeable on?