Here's an example:
Alex: (posted at 5:26pm) "I love hot dogs with chili and cheese."
Sam: (posted at 5:30pm) "By the way, does anyone know what Jim Henson's favorite food was?"
Tracy: (posted at 5:30pm) "Oh I love them too!! Nothing beats a chili dog!"
Tracy saw Alex's post and replied to it without using the "quote" feature, since it was the most recent post. But then she didn't realize Sam was replying at the same time and his post slipped in before hers, breaking up the continuity of the direct reply. This is a classic Ziffel because of the break in thought sequence! (A weaker ziffel pulled would simply be slipping in before someone's reply but having no consequence to the flow of thought. Eg. If Sam had instead posted. "I love those too" he would have still pulled a ziffel on Tracy but it wouldn't have interfered with the flow). I have done this a lot in the past and D'Snowth brilliantly coined the term ziffel to describe it!