I upgraded successfully LR4 to LR6 on OSX 10.8.5 (MountainLion) on a MBP.
LR6 runs so far.
But always after updating to LR6.5 I could not mor start LR6.5.
After installing LR6.0 again all runs fine.
Seems to me that LR6.5 is not compatible with Mountain Lion!
Any other victims?