The series lightbulb principle has been used for various troubleshooting techniques. It's sometimes used to find problems in TV sweep circuits which are blowing fuses by placing a lamp in series with the B+ supply.
It's even been used as a poor man's overload device on model railroad controllers -- A car headlamp bulb in series with the track supply which will illuminate in case of a derailment and short circuit.
In this case though, I'd isolate the circuit at the panel and use an ohmmeter to check for obvious shorts. If a low-voltage check didn't show up anything suspicious, then I'd put a megger on it.
