Here's the scenario:
I have a multi-user server (Ubuntu 9.10 x64 server), which currently has three primary users (I'll call them user0 (me), user1, and user2). As the server admin, I need to have write access to both user1 and user2's respective home directories. Consequently, to reduce the number of times I have to invoke root privileges, I added myself (user 0) to the user1 and user2 groups, and then recursively adjusted permissions to give myself the proper access:
groupmod -aG user1,user2 user0
chmod 770 -R /home/user1
chmod 770 -R /home/user2
Much to my dismay, it initially appeared that my permissions-mangling had had no effect; trying to ls or cd (as myself) into either of the /home directories in question consistently produced a "Permission denied" error. Some 8 hours later, however, everything seems to have happily settled into place, and I can access both directories without any problems. I have not restarted the server during this period, or done anything particularly intensive (other than ssh-ed in a couple of times, which hardly counts as intensive).
Anybody have any ideas as to why my permission-adjustments were delayed, or what caused them to suddenly take effect?