in example:

I have flashed 2 times my n900 in 2 months.

1) Playing with extras-devel without realizing the risks, i have installed an app who mess the sudoers file. some hours later, the developer fixed the bug, but my n900 was unable to install new apps, write in /tmp and receive sms.Time to reflash

2) Playing with the configuration settings, i have touched a lot things like tracker, init.d, /etc, installed certain daemons manually, and then i realize that i dont know the way back . My n900 was slow and the battery drain fast. Time to reflash.