The difference between being funny and being a jerk.

I arrived at work to overhear a co-worker talking about the end of a movie currently in theaters.  When I realized what was going on, I said in a voice a bit louder than I had intended, "Dude!  Spoiler alert!?!"
In a voice a bit louder than I'm used to hearing, he replied, "Do you even know what the heck I'm talking about?"
"Yeah!  Ender's Game!"

And suddenly he became very quiet.

It was awkward.  I hadn't meant to sound as jerky as I did, and now he was feeling bad.

A minute or two went by.  "Hey," I said in a much more volume-appropriate voice, "I only knew which movie it was because I've already seen it, so I already knew how it ended."
"I was trying to be really vague.  I didn't think anyone would know what I was talking about."
"Yeah, and I was trying to be funny.  I am very often mistaken."