double take

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A delayed reaction to an unusual remark or circumstance, often used as a comic device.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun colloquial A take, commonly used as a comical reaction to a surprising sight, in which someone casually sees something, briefly stops looking at it, realizes what it is, and snaps attention back to it with an expression of surprise or disbelief.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a delayed reaction indicating surprise