My opinions on Commodore's greatest mistakes:
1. Cancelling the A3000+. This would have introduced AGA in mid 1991 instead of late 1992 and kept the Amiga's foot in the door during the rise of VGA.
2. Botching the deal with Sun to use the A3000UX as an OEM low-end Unix workstation. Should have been easy money.
3. Replacing the A500+ with the A600, which was designed to be the botton-of-the-line entry-level model to complement the A500.
And there are of course a gazillion others, but without these 3 I think they might have lasted longer.
