I recently did the classic "congrats on the pregnancy" thing when the person wasn't pregnant.
I went into work for a visit (have been on maternity leave for like 6 months) and said to a collegue "omg you're pregnant!" She said "no", another collegue tried to cover for me and made a comment like "she only said that because you look like you're glowing". I was so convinced that she was pregnant that I mortifyingly go on to say "no, no you look pregnant. Are you sure you're not pregnant?" Of course she went on to say no again and I felt sooooo bad. I still have no idea how or why I had to insist that she was pregnant lol.