If you have a problem with the third act, the real problem is in the first act.
— Billy Wilder, 6 time Oscar winning Writer and Director
Problems in the end of your story are almost always the result of problems with the beginning of your story.
Problems with a client towards the end of project are almost always the result of problems in your initial brief.
As in art, so in life.
I recently got myself into this exact problem by hurrying the briefing process with a new client. Part of the problem was that I largely accepted their brief as they presented it, rather than creating my own brief and presenting it back.
Doing this takes time, but it ensures that you actually understand what they need and what work is involved. Thus you can accurately scope and budget the job.
I didn't really do either of these things, and I also glossed over their complete lack of experience in video creation.
Needless to say I experienced 'third act' problems* that I then had to solve on my own time and dime.
*Problems such as multiple, entirely different deliverables, terrible client script I had to entirely re-write, too many iterations and levels of sign off etc. etc.
Ah well, the best way to learn a lesson is not to, at first.