I thought it was always "in the bag, last couple of redlines to get sorted, but that'll be sorted by the end of the week, no worries."
But yeah, 2 weeks to implement, then it sits there whilst they sort out some overlooked complication
(One contract job I did a while back, the first thing the client asked me for some was some documentation. The forwarded email trail explaining exactly what documentation also contained an email sent two weeks prior, apologising for the delay and saying 'their programmers' were working on it.)
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.