It seems like some of these suggestions are heading more toward "How to annoy your players" rather than "How to freak out your players." Anyway...
368.) Have the wizard's familiar begin sprouting tentacles at random times. Then have the wizard wake up one morning with an eye on the palm of his hand.
369.) Find a Wild Surge chart. Then have the party unknowingly walk into an area of wild magic, where every spell creates a surge. This works best when using the surge chart from the 2E Tome of Magic.
370.) While the party is away from home, have a player's mother slain by a demon, who then impersonates his mother and dominates his kid sister. The sister is commanded to lure lecherous men to their deaths by enticing them with forbidden pleasures. When the player returns, allow him to investigate the dissappearances of some dirty, powerful old men and come across clues of gruesome deaths. Find some excuse that prevents the character from meeting his mother. Then have his sister come find him because "Mommy wants to talk to you."
371.) Rig all of the lights in your room so that you can switch them off with a remote control switch. Then set up some very bright white lights (maybe something like a camera flash) behind the curtains, also controlled by remote switch. Put a tape with a booming crack of thunder in your stereo with surround sound, and crank it up. Make sure the players don't know about any of this. At some dramatic point in the session, turn off the lights in the room, then immediately hit play on the stereo and trigger the flash behind the curtains.