A little background, from Wikipedia:
Gremlin is an English folkloric creature, commonly depicted as mischievous and mechanically oriented, with a specific interest in aircraft. Although their origin is found in myths among airmen, claiming that the gremlins were responsible for sabotaging aircraft, John W. Hazen states that "some people" derive the name from the Old English word gremian, "to vex". Since World War II, different fantastical creatures have been referred to as gremlins, bearing varying degrees of resemblance to the originals.
One of the most vexing gremlins I know of is a fairly recent incarnation, the Email Gremlin. This little fellow grabs your email right after you send it, and sits on it for a period of time that can range from 5 minutes to several months. Have you ever encountered him?
Here's how he does his work: You and your correspondent are both at work, shooting emails back and forth, having a medium-temp conversation, when suddenly your friend just drops out, usually after you've asked a question or made a clever joke. Then you're suddenly sitting there hitting 'refresh' over and over again and wondering "Is (s)he mad at me? Did I say something offensive? Is there an emergency? I thought we were cool..."
After a suitably tense interval, the missing email(s) start(s) popping up in your inbox, with time markers showing you that you actually got a prompt response to what you wrote. Somewhere an email gremlin is rolling around on his Disney-esque butt and laughing at you!