It just happened again, this time dmesg only contains an ACPI Warning, so I doubt it's Compiz-related:
kernel: [20920.389627] ACPI Warning: \_SB_.PCI0.PEGP.DGFX.MXMI: Excess arguments - needs 1, found 2 20090521 nspredef-287
kernel: [20920.389671] ACPI Warning: \_SB_.PCI0.PEGP.DGFX.MXMS: Excess arguments - needs 1, found 2 20090521 nspredef-287
pulseaudio[32661]: pid.c: Stale PID file, overwriting.
It just happened again, this time dmesg only contains an ACPI Warning, so I doubt it's Compiz-related: PEGP.DGFX. MXMI: Excess arguments - needs 1, found 2 20090521 nspredef-287 PEGP.DGFX. MXMS: Excess arguments - needs 1, found 2 20090521 nspredef-287
kernel: [20920.389627] ACPI Warning: \_SB_.PCI0.
kernel: [20920.389671] ACPI Warning: \_SB_.PCI0.
pulseaudio[32661]: pid.c: Stale PID file, overwriting.