Someone in this thread mentioned the police and how they usually don't enforce the law re lights on a bike. I actually seen the Dutch police control groups of bikers ... if it's dark and your light isn't on or not broken, you can get a ticket. You'll read about that the next day in the paper.
yeah, in Germany you're pretty likely to get stopped by the police if you ride downtown at night without lights or happen to ride by a cop --- although they tend to give warnings rather than tickets i think (i ride with lights like 98% of the time so i wouldn't know)
and technically in Germany you can get a ticket riding during the DAY without a light based on the idea that anyone riding could be out long enough to be in the dark so must have lights. although there is an exception for "performance" or "racing" bikes which are currently defined as less than 12kg i think -- although this is an old law assuming only light road bikes were "performance" and doesn't technically include freeride and downhill bikes - "performance" bikes are exempt during the day from the light-requirement.