If you are working on a fixed price project, there is a very real risk of scope creep. You need to actively manage client expectations so both of the parties end up happy with the transaction.

• Make sure you are tracking as tickets are getting closed, you don’t start getting new tickets being opened.
• If you are 50% of the way in the project, 50% of the tickets have been closed and 50% of the milestones completed.
  (And have a client sign up on them)
• Always be talking to the client, to understand their estimate of what percentage of work has been done.
• When you are 2 weeks away from projected completion date, get a confirmation of the hand off date from the client and
  ensure that the both of you agree with what current state of project looks like, and that the target date looks achievable to both of you.

Fixed price projects are riskier but with managing expectations and with a bit of give and take you can ensure both you and your client end up happy with the project.