I think I know at least one situation when this happens. And this observation also relates to the posts with which I opened this thread.

With bme replacement (how was it before???), my n900 is quite sensitive to a partial-contact made whilst (ever so carefully) inserting the wall-charger connector.

With usb mode installed one can readily see this. If you make a "hesitant" contact, usb mode will display to request mode selection. The device will charge but in fact thinks it is connected via usb and not the charger. This likely happens because the hesitant (partial-) contact does not register the expected current and so defaults to the lower.

I get the persistent green light when the charge completes under this scenario. Don't know if it always happens however. So far, the persistent green led has not occurred when the wall charger was seen at the start or when I used usb mode to set the mode as "usb peripheral mode" after a partial-contact connection was made (whereupon the n900 sees the wall-charger).