I'd like to reassure you all that these incidents are very rare...but I'd be lying.
There's an inordinate amount of time hanging around the car parks waiting for the track to reopen after either a multi-car accident (usually caused by one car dropping fluids), or multiple independent incidents (usually driver/car failure), or fatality (usually a biker vs fluid vs barrier incident). Then you can have up to an hour to get out of the overflow car park and back onto the track.
Some weekends are completely clear...but they're in the minority...so you need to have a back up plan on how you're going to spend the day instead of hanging around for 4 hours during a closure.
We tend to do early morning (8-10), and late afternoon sessions (5-7), and go driving around the region for the 'nutter time'. We can still get 15-20 laps in those 4, quieter hours.
We also turn up on the Friday afternoon so that we can get our tickets sorted and do a few laps before we check in to the hotel.
The German bank holiday weekends are the worst!