gnu linux update question

[Petrus de Calguarium]

It is common knowledge that one does not need to reboot for updates to take 
effect in GNU Linux.

However, in actual practice, this is not so. I could cite many examples, but 
this should suffice:

On Sunday evening, I installed a new updates-testing version of mesa and then I 
suspended the machine for the night. The following Monday morning (yesterday), 
I resumed the machine and suspended it again around noon. I again resumed the 
machine at about suppertime and _powered_ _it_ _down_ about 2 hours later. An 
hour or two after that, I powered it back up and the mesa testing update turned 
out to be bad and I was not able to log in. I did not know which program was at 
fault, because the bad program had been installed over 24 hours prior, but was 
only showing itself to be bad after a power off.

Could someone explain how reboots are not needed in Linux for updates to 
_take_, given the evidence to the contrary.