Similar to the observer effect in physics, the very act of one person saying they will attend an event can cause all other people who were going to attend to change their minds. Has been known to ruin entire parties.
<Dave> Man, this party is going to be awesome, are you guys coming?
<Adrian> I'll be there
<Joe> I'm coming!
<Adrian> Actually wait, I just realised I had something on
<Dave> Damn, the Joeserver effect!