http://en.wikipedia.org/wiki/Data_degradation ("Data degradation in memory can occur when the small electric charge of a bit in RAM disperses, possibly altering program code or stored data. The hypothesis that semiconductor RAM may occasionally be altered by cosmic rays[2] is also known as soft error.")
Private keys can be subject to bit flip errors, so individual wallet can lose coins if there is no backup to restore the wallet from. In that case it is usually also possible to recover the key by trying to undo the bitrot in bruteforce manner, usually the search space is rather small.
As for the rest, the network is embarrassingly resilient (faulty node is no different from evil node).
