I noticed this in Crimson Cove. Sometimes, when you begin attacking a group of Imperials, one or two of them will momentarily disappear as they notice you and turn to attack. While "blinked out," a targeted NPC will be unaffected by spells such as power extraction. Why does this happen?
"Argonians have fat, scaly tails." —Rissa Manyclaws.
"Once upon a time there were three sisters: Delicious, Delightful, and Disgusting. Now, Delicious and Delightful were both very pretty girls..." —Brendalyn Jurarde.
"I smell to the nobility." —Indrasa Avani.
"A bargain with an animal is not a contract made." —Haderus Atrimus.
"Redguard makeup for sale. Free samples. Secret ingredients. Unique application method. Lots of satisfied customers." —The Mudball Goblin (aka, Cognac Vinecroft)
"Your armor looks like underwear." —Shuns-the-Knife.