The Mac mentioned in the best possible way by the smartest character on Stargate Atlantis

[Derek Chesterfield]

On 21 May 2007, at 07:04, Matt Johnston wrote:

>> Since I've started using this procedure, I haven't had a problem  
>> waking up. Step 3 is the critical one. If I perform the same  
>> procedure but omit logging out, the computer will more than likely  
>> refuse to wake up, requiring a hard reboot.
>
> This is, of course, not the norm.
> [...]
> If logging out makes a difference then there must be a software  
> reason for it. What processes are you regularly running which are  
> hanging on sleep?

In my case, at least, it was simply the external monitor being  
present at sleep causing the problem. I'd operate my PowerBook in lid- 
closed mode, put the computer to sleep, disconnect the screen etc.,  
and leave. If I connected the computer to the screen before waking  
it, I was good, but if I tried to wake it without the external  
monitor, I'd have a problem about 1 in 3 sleeps. The screen would  
wake, I'd see the cursor - which was moving with mouse input, but I'd  
never get the password prompt. [Prompt for password on wake is  
enabled - never tried to see if disabling that fixed the problem.]  
When I had the problem, sometimes I could close the lid, wait for  
sleep, and try again, and get a success. Other times, if I tried  
repeatedly, it would eventually fail to sleep.