Battery dead or human error?

Come gather 'round, Messrs. and Mses. Fixit! I have need of your brains…

My friend's got a 2006 MacBook whose battery has stopped charging (black X on battery icon). The System Information app reports a Cycle Count of over 400 and Condition: Needs Replacing.

The thing is, I have previously opened up the MacBook to clean the dust inside (unplugging the battery connector from the logic board in the process), and I am not sure if I put everything back together properly…

My question is: could it be that the battery did not simply gave up the ghost, but fell victim to my carelessness? How do I know if it is safe to put in a new battery?

Any kind of help is greatly appreciated~