It doesn't have to be something serious, but it can if you want it to be.
I got the idea from someone's (darn can't remember their name) post about romance. Hope they don't mind.
Mine would be that my best friend won't ever let me forget about what happened last summer. There was a mishap with some dye and I ended up with purple hair for a month. :x People seemed to like it, though.
She also refuses to let go of the fact that she thinks her brother is "interested" in me. I: She quite likes the idea, though, because she wants me as a sister-in-law. She actually made a 'suggestive' remark about me and her brother (pertaining to the future and her belief that we'll end up married) in public. I smacked her. (: