In 5-ish years of consulting I only had this happen once. BigCo hired LittleCo to produce one piece of a very, very large system with many parts. I was consulting for LittleCo. Our part was on schedule, but no other parts were. They couldn't let us go because they needed support for initial roll out, but they didn't have anything for us to do.

We did some make-work projects, optimizations and stuff. We also picked a random technology stack per week and spent a day building "something" to learn those tools. Also, we played a lot of video games. This period lasted about 3-4 months.