The Tooth Fairy Is a Pain in My ...
Example 1: The tooth fairy sometimes has a glass of wine and watches Top Chef and then forgets that there is a tooth waiting to be claimed, resulting in the tooth fairy sitting straight up in bed at 4 AM saying, "Sonofa ...! I forgot again!" Then she sneaks into the small child's bedroom in the manner of an ill-tempered, uncoordinated, slightly hung-over cat burglar.
Example 2: Often the tooth fairy will get distracted by things like folding laundry and soccer practice and will not remind Mr. Tooth Fairy that he needs to make sure he has cash. So the lucky, lucky child who just lost a tooth gets a savings bond! Yay!
Example 3: Once the tooth fairy only had $20 and so now her children have totally unreasonable expectations.
Example 4: If a child accidentally swallows a loose tooth while sleeping and doesn't even notice it until after lunch, then the tooth fairy has not shirked her responsibility for once.