I recently found a single-character bug that had been around for months, and was even in production. In a stored procedure, there was something like

SELECT COLUMN1, COLUMN2, COLUMN3
COLUMN4, COLUMN5 FROM TABLE1 WHERE ...

It wasn't discovered, since COLUMN3 and COLUMN4 weren't being used. Once I started implementing a feature that used COLUMN3, trying to retrieve it using the stored procedure resulted in a no such column exception, and once I looked at the source code for the stored procedure, it was obvious.