Exposure

When exposure is mentioned on Charmed, it is usually referring to human society discovering the existence of magic. This was something which all supernatural forces - both good and evil - had to avoid at all costs, given that there are highly dangerous consequences should mortals discover that magic was real. A testament to this would be how one such exposure set off a series of events that ultimately led to the tragic, irreversible death of Prue Halliwell, the oldest Charmed One, which in turn broke the Power of Three altogether. Though the Power of Three was later reconstituted by the revelation of the youngest Charmed One, Paige Matthews, it was still a lesson that had a dramatic impact on the rest of the show and on the sisters' lives, one that they especially took to heart, given that the cost was so dear: the loss of a sister.

Secret Keepers
...Many more...
 * Andy Trudeau
 * Darryl Morris
 * Victor Bennett
 * Bane Jessup
 * Henry Mitchell

Notable Occurrences

 * Season 2, Morality Bites. In an alternate future, Phoebe exposed magic and brought back the Witch Trials by killing Cal Greene with her powers. This timeline was erased.
 * Season 3, All Hell Breaks Loose. The world discovered magic because Prue and Piper were filmed vanquishing Shax. Tempus later reversed time.
 * Season 6, Chris-Crossed. Future Wyatt turned evil caused by The Elder Gideon and exposed magic to the world while ruling over San Francisco. This future was altered.
 * Season 6, Forget Me Not. The sisters expose magic to lure out the Cleaners in an attempt to get Wyatt back, who had previously conjured a Dragon. The Cleaners erased the expose.