This was actually a few years back but it's the worst I've ever done.

I had a coworker who happened to live in the same apartment complex as me. We both had pet rats at the time and she ended up calling out of work when one died because it had been a sudden death.

So I made a little care package and brought it over and we were hanging out and I was trying to make her feel better. I noticed she had a big green floral couch in her living room. This was student housing so it was furnished and we had already discussed how I had an upgraded unit and she didn't. She had complained about the ugly furniture.

So I turned to the couch and said, "Wow! I see what you mean about the old furniture being ugly." It wasn't even ugly. It was like a kitschy vintage couch. I just was looking for something to say.

There was a long pause and I immediately knew I had fucked up. She goes, "Actually, that's mine. I put the other couch on the porch."

Of course I profusely apologized and she said it was fine but damn. I was mortified my attempt at being nice turned into me insulting her things.