Is it ironic if you err when you err?
To err is human. To err instead of er is hellish — to an editor, anyway.
Err is a verb meaning to make a mistake. Er is an interjection expressing hesitation or uncertainty, similar to uh or um. Examples: Fourteen times six is, er, 84. The capital of Illinois is Chicago, er, Springfield.
When you're not sure what to say, say er. Otherwise, you err.
I'll let the writer off the hook, though. After all, to forgive is devine, er, divine.