He said AC not DC - hopefully that was a typo too!
Maybe it was Guy Fawkes day? (insert your local pyrotechnic occasion). Just putting 5V on the cell is dangerous enough. Though the protection chip should have helped.

My story:

I found the n900 turned off - probably low battery. But it would not come on with the charger connected, even though the charging light glowed. Charged a while, still would not start. Eventually I took the battery out and measured 3.45V, so presumably it had indeed been charging.

I put battery back and it worked. Why would I have to remove & replace battery to get it back on after running flat?