Was showing the new guy how to write a fairly simple database query with a couple of joins.

Spent 3 hours trying to figure out why it didn't work.

Finally discovered that I had randomly chosen one of the 3 records (out of a possible 15,000+) that had leading white space.

Ctrl-Z back to the first query I wrote (3 hours ago), and it works perfectly.

New guy learns a more valuable lesson than I originally intended.