I can't believe there is no simple solution to this (because it seems potentially a very useful feature to me), but so far I couldn't find any.
I'd like to be able to (temporarily) turn off my own write permission to the filesystem I'm using. Let's say, I'm logged in on a sensitive system where all I want is to view files but I want to avoid doing any changes by mistake.
A lighter version:
If all the write permissions I have are due to the groups to which I belong (I disregard now my home directory), can I turn off these permissions by somehow temporarily removing myself from these groups (newgrp only changes my default group but not the whole group list).
Of course, in either version I'm asking about doing this without the root privileges.