It's NOT a bug OR a glitch. The reason this happens it when you plug in your N900 to charge it switches on so that it can check the battery level and keep flashing the LED (and monitor the whole charging process).

BUT it doesn't fully switch on, like it would when you press the on switch.

So you leave it for a few seconds to switch off and then press the power button, this was the same with the N800 and N810, and also my N95.

Best way is to switch on your device, THEN unplug the charger.