Climbing out of Houston on a Sunday afternoon:

Houston Center:
"Citation 123, please see if you can contact Challenger ABCD on 128.17 and have them contact Houston Center."

[A few minutes pass.]

Challenger ABCD:
"Houston Center, this is Challenger ABCD checking in at flight level 360."

Houston Center:
"Roger, Challenger ABCD. Descend to FL 240. What was the last frequency you were on?"

Challenger ABCD:
"It was New York Center 132.05. They never handed us off."

Peter Serodino
via e-mail