Yellow & Red Tulip
This is pretty cool and not normal. This phenomenon is known as a sport or a chimera. It happens when a plant produces an otherwise identical offspring that has a uniquely different appearance, such as color. It is caused by a genetic mutation.
Thank you very much for the explanation. I looked up Tulip Chimera and, sure enough, found several example of this mutation. It is cool!
Oh, I guess that brings up another question: is the mutation permanent and my wife should expect her "mutant" to come back next year?
This type of genetic mutation is considered very unstable. It is unlikely to appear this way again next year. Here is a nice brief article on sectorial chimeras. http://gardenprofessors.com/when-half-a-flower-is-the-wrong-color/
Bummer! My wife was all excited about having a mutant garden. Thanks for the responses.