I have my N900 now for nearly a year, had to reflash 2x.

First time was after a long time, experimenting a lot with it in the first months, installing packages (sometimes from extras-devel), deinstalling them again ... at a certain point dependencies became inconsistent, several apps started to behave strangely, I got especially problems in the sound subsystem... so I decided that it will be much easier and much less work to just reflash than to analyze in detail what is going on.

The second time I accidentally removed the omap_hsmmc kernel module (don't ask why I did this...). The device rebooted instantly, and the filesystem was f**** up beyond hope. This was especially nasty because I needed the phone urgently the next day, and it was late in the evening. After two hours, I luckily reflashed successfully and had restored all data I needed.