I had this happen also with a particular customer once. They were a large financial institution who bought our product to replace their previous software package that had been way past EOL.
Because they already had software to do this task, my boss assumed that determining the custom requirements for them would be very straightforward, and would only take a week or two. We ended up spending the next 3 months in very tedious conference calls preparing requirements documentation that their obnoxious internal processes required.
Just when we finished that, the customer came back and said, "That's great. Government-Alphabet-Agency gave us a hard deadline to switch to the new system in three weeks." By that point the scope of the project expanded to about 3 months of work.
We still managed to ship something for their deadline, somehow, and we're still providing maintainence for that, oddly enough.