@vi_

I also have (had?) that with my dev-N900. It happened during the time when it bricked and I desperately tried to fix it.

Two (wild) thoughts:
1. N900 gets electrostatically charged, making it do weird things.

Reason: during my attempts to flash (or even to set R&D mode) on that N900, I booted it many times by connecting the charger without a battery in place. It behaved very weird. It entered flash mode (NOLO), but the computer didn't notice a thing. Also the microSD card didn't work.

After leaving it a couple of months in a drawer, without the battery, I managed to unbrick it using the same method I had tried before (OK, this time I used the set_omap_atag or whatever the name was when booting from the U-boot console).

After unbricking it, charging worked again (shaky, but it did), and I was able to set R&D mode using the flasher (so USB works after all).

2. If you take out the battery while not really off, but ACTDEAD, it could be that reinsterting the battery makes it boot automatically (e.g. so that you don't miss an alarm!).

Just two wild guesses. When I have the time I will experiment again with that particular N900, but AFAIK it does not boot anymore automatically when inserting the battery.