The problem was displayed as Undefined Set: Problem 1. Both before and after I tried the problem, the following appeared below the Preview, Check and Submit Answers buttons:
You have attempted this problem 2,000 times.
A bug? A feature? Humor? 
WeBWorK Main Forum
You have attempted this problem 2,000 times.
This forum has a limit to the number of forum postings you can make in a given time period - this is currently set at 10 posting(s) in 1 day