Climbing out of Houston on a Sunday afternoon:
"Citation 123, please see if you can contact Challenger ABCD on 128.17 and have them contact Houston Center."
[A few minutes pass.]
"Houston Center, this is Challenger ABCD checking in at flight level 360."
"Roger, Challenger ABCD. Descend to FL 240. What was the last frequency you were on?"
"It was New York Center 132.05. They never handed us off."