Biggest blunder, huh?
Okay, it's story time. Years ago, I was dating this girl (full lesbian) who had some self-esteem issues. She was complaining about how no one found her attractive. I said "that's not true" and proceeded to mention that, of course, I found her attractive. I also mentioned several of my friends who'd comments on her attractiveness, including a male-friend's fiancée.
My girlfriend was shocked by this - she had no idea that this girl was bi or thought she was attractive. She asked how I knew, and I explained that she (fiancée) told me so when I started dating her (my girlfriend).
Anyway, about a week passed, and then my male friend walked in on his fiancée and my girlfriend having sex. Thus exposed, my girlfriend dumped me to be with his fiancée; she broke off the engagement to be with my now ex-girlfriend.
So there's my blunder.