A startup (series A-B) I worked for was basically a thin wrapper providing physical goods around the core features of a delivery SaaS (series A) - our core features were so entwined with theirs that when they went belly-up, we panicked and bought their source code for an ungodly sum. Then we took over hosting their product for just ourselves, and so had to learn how to run _their_ software too. It was written in a different language than most of our engineers knew, and had pretty much no documentation. We didn't know enough about their software to fix new bugs easily as they came up, and didn't have enough `people * time` to figure out enough to fix the old ones.

Entwining our business with theirs was a good way to get off the ground. But when they crashed they nearly took us with them.