Discworld is a hive of activity - things are always being added, changed, updated, upgraded, downgraded and generally progressing.|
As such, there may be times when you encounter an error in the game. A runtime error means that something in the code is faulty, and the general message that you will receive when one occurs is:
"A runtime error occurred. Please make a bug report describing what you did when it happened."
A 'bug report' is a way of notifying the coder of the thing that runtimed that it did, in fact, runtime. The help bug helpfile explains how to file a bug report.