Some of the other members of the #maemo IRC channel also said that it was my fault that two of the N900's went into an infinite loop on boot-up, but we couldn't figure out exactly what I was doing to cause this. If you could help figure this out, it would show that you're not just posting unhelpful comments here.

These are the possibilities I thought of:
- Is it because I'm installing weird software? Three times out of four, I did not enable any new repositories, but I guess it could be that software that has gone through extras-devel, promoted to extras-testing, and has had enough votes to go to just plain extras, could still have some weird bug. (One time the error triggered because I enabled extras-devel to install BlessN900, but that time there had been no system slowdown or weird error messages.)
- Is it because of a bad SIM card? Forum members said that this was next to impossible (to have this error, not to have a bad SIM card)
- Is it because of a bad microSD card? Forum members said that this was next to impossible (to have this error, not to have a bad microSD card)
- Is it because I created a symlink from ~/ to ~/MyDocs/.documents? "~/" is on a Linux filesystem (ext3 or ext2 or whatever) while ~/MyDocs/.documents is on vfat, so maybe that blew it up. But if I recall correctly, the error happened even without the symlink.

Any advice would be appreciated.

@MiK546: I did not enable extras-testing/extras-devel out of fear, but the error happened nonetheless.

After a while I got sick and tired of not installing anything out of fear, and decided to do it. As far as I can tell, it doesn't make a difference whether or not I enable extras-devel/testing. (Btw, I immediately disabled extras-devel/testing to make sure nothing stable would get accidentally updated to a newer, buggier version.) I think BlessN900 is extras-testing, not extras-devel. I also installed Midnight Commander, which is essential for browsing and retrieving files over ssh; MohammedAG says it's rock-solid stable, and I believe him.

@Matthew Maude: I, too, suspect something installed but can't tell what it is. All the files I transferred to the device were backups from the device. I don't have any downloads that "aren't from the stock catalogs", as I had mentioned. It seems rather unlikely that a virus would have gotten onto my Kubuntu system and then transferred onto the N900 (which doesn't even use a x86 processor), but I had already followed your suggestion of "you'll have to re flash, make sure you use upto date os and emmc" several times, as I noted in my previous posting.

@YoDude: Yes, I do use a SIM. Do you think it could be because of a bad SIM card? I have been using the same SIM in my old phone for about six years now; I'm not sure I want to change it because I would have to change my phone number (among other hassles that outweigh the disadvantage of just not using a N900).

@pantera1989: Thanks for the suggestions. This is probably the most helpful reply so far. On reflection, it is conceivable that it's the MicroSD, since:
- if the bootup sequence involved checking something about the MicroSD, an error might launch it into an infinite loop
- if this problem occurs, it's easy to remove the microSD card; unlike the SIM card, I don't have to remove the battery to remove the microSD card
- I'll try flashing different versions of PR 1.2, although chances of success are slim. When I tried to flash just the firmware, the problem remained; I had to flash the eMMC also in order to get a working system again, so I think the problem is not in the firmware. But it's worth trying.

I guess my point is that I don't think it's just luck. It looks like there's a problem inherent in the N900. Other forum members have told me that they have been using the N900 for a long time with no problems, but that doesn't really help me. Even if I'm able to use the N900 from now on with no problems, until I can figure out what caused the problem I cannot rely on it: my phone might go wonky on me at any time, and in this day and age of relying on cell phones, this is not an acceptable situation.