Buffy: The Vampire Slayer - Glasses vanish and Reappear
In the season 2 episode phases watch between the 36- 37 minute mark when willow enters the library to tell Giles and Buffy that Oz is a werewolf. First Giles isn't wearing his glasses then they appear only to disappear again when willow grabs him. In a rear shot of Giles you can then see they've reappeared again only to disappear once more when he turns around.
Season 2 episode of Buffy phases