Pawn movement

I had a pawn on e4 and the opposing king directly in front of it on e3. Computer suggested moving the pawn diagonally to f3 without capturing a piece. f3 was open. Is there some sort of unique rule with how pawns can move when blocked by a king?

Looks like this was a situation where the special move en passant was available.

You can learn all about it here: https://www.chess.com/terms/en-passant

Probably no way for you to go back and check, but maybe the pawn on f2 moved to f4 and the computer wanted you to capture "en passant".   Pawns don't move diagonally unless they are capturing, but you may not have noticed the capture if it was en passant.  And just in case that's new to you, I'll get the article...

Day late and a dollar short again...