I don't know if this is general knowledge, but thought I'd mention it.
I just got around to upgrading my dual 1.8 Ghz PowerMac to Leopard. At the end of the process got a blue screen, not to be confused with Window's Blue Screen of Death...or could they be related?

Anyway I momentarily envisioned rebuilding my MacOS, but fortunately a little research revealed this
Apple Support link which fixed the problem using Option A in the link.
An application called the ApplictionEnhancer.bundle was the culprit, which Leopard does not like, and which can be installed for several reasons, one in my case (I think) was installing logitech drivers to run a logitech mouse.
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"Hey, hey, hey, they're playing our song. Let's go fight some monsters!"
MBP, 2.2 GHz intel Core 2 Duo, 4GB Ram, 8600M GT (128MB VRAM), 32b Vista