What makes Clippy such a good prank?

Clippy is brilliant because it taps into a shared cultural memory. Anyone who used Microsoft Office in the late 1990s or early 2000s remembers the original Clippy -- and dreads his return. The prank works on two levels: the immediate annoyance of persistent pop-ups that cannot be closed, and the deeper horror of realizing that Clippy has somehow come back from the dead.