- Forgot to bring dismountable battery light.
- Dismountable battery light was stolen.
- Empty batteries.
- Light bulb died.
- Broken cable or other disruption of electricity from a dynamo to the lights.
Number 1 to 3 can easily be avoided by having a fixed, dynamo-powered light.
Now, some inventors came up with a simple solution for number 5: integrate the dynamo and lamp. The product is called Reelight and here's a nice review of it. I personally oppose flashing lights on vehicles (except emergency ones), but Reelight has developed their product so far that the newest version can also produce a steady light. Progress here you come!
In related news, there's a fun gimmick which could make your bike ride even safer: self-illuminated pedals. Looks good, but I have no idea how much it really does for safety. I hope it doesn't have such a negative, traffic-disturbing effect as those flash-lights do.
PS: Of course, you can also solve the transmission problem by simply using robust 2$ two-core cables on your bike, instead of 0.20$ single-core plus frame built-to-break wires.
Update: Judging from other reviews the reelight doesn't seem bright enough to actually show you the way. That would mean you still need a dynamo if you safely want to avoid potholes and other crap on the street. I decided a pair and test them myself (on a friend's bike), as soon as I am back to Canada.