What is a runtime error, and (if not obvious from your reply) what are the most likely causes of a runtime error?
“Runtime error” is just an error in a program that occurs at the time the program is run (as opposed to when it’s compiled or developed). Almost all user-visible errors are runtime errors. Without a lot more context, we can’t be any more specific than that.
A runtime error is an error that occurs at runtime - that is, not at compile or link time.
Anything can cause runtime errors, but usually they are the result of bad programming.
Care to share a specific example?
ETA: TimeWinder and I seem to be on the same page about this 
Ah…I see. Thanks for the explanation of this general term.