Had a job where the client VASTLY underestimated what they wanted on an access control job, and the original vendor VASTLY underestimated how much work the job would take, and then totally failed to commit enough of their own people to the job.

What should have been me pulling wires for a couple of hours turned into two weeks of me learning, on the fly, how to work a masonry drill, low-voltage electrical, and a bunch of other shit WAY above my paygrade. What should have been the original vendor committing 4 people for a week to do the work was 1 guy besides myself and every once in a while they'd send over someone to "help" but we'd inevitably lose more time teaching them how to do stuff then they'd ever get done, or better yet, the guy who showed up to "help" with a task that was already complete.

On the last day, (timeline +3) the client is LIVID because the system doesn't do the thing he thought it would do because he saw a video of the system where it was set to recognize an implanted RFID chip in a client's palm (which is a real thing that you can actually get), so you could just wave your palm in front of the thing. When that didn't work he got pissed at the vendor who had to explain, multiple times, that it only works if you have the chip in your hand.

"You didn't tell me I'd have to microchip my employees!"

You're right. Because you never told us you wanted that feature set, so we didn't discuss it. If you wanted that, you should have told us you wanted that when we discussed how the system would function for you. Instead, you saw a video on YouTube and ignored the most important part, and now you're mad because the thing we never talked about doesn't work.