Accidentally wrote 'happy birthday' on a sympathy card at work

Get your resume tuned up.

Was it his birthday, perchance?

Oh dear. Maybe just stop by and offer your sympathy and let him know about your oversight. Shit happens.

Send an edible arrangement. Yes, dig deep in your pockets and pay for it. You really screwed up in a big way.

Of course they read the comments. Maybe not right when they get the cards, but usually at some point.

I once accidently choose the smiling/laughter face on a facebook post when someone said their pet died. Meant to hit the sad button and didn't realize til afterwards when two people brought my name up asking why I thought it was funny!
Then it became funny actually!

this joke has already been done before.

Maybe he’ll appreciate the humor.