1. Mention someone in a post. Example: @Kyumorph
2. Edit the post immediately. Should look like this: [MENTION=52228*]Kyumorph[*/MENTION] (w/o asterisk)
3. Replace their username with something else. Example: [MENTION=52228*]Morphkyu[*/MENTION] (again, w/o asterisk)
4. Save the post. They'll get the notification, and see a name that's not theirs.
5. BAFFLE YOUR ENEMIES AND AMUSE YOUR FRIENDS
You could also do this:
This reduces risk of them seeing it before edit.
There's also invisible mentioning.