Biggest mistakes you’ve seen on a construction project?

I’ve been around a few jobs where things went sideways, and most of the time it could have been avoided. Poor planning, wrong measurements, rushing the work they all seem to pop up more often than they should. One thing I’ve noticed is that a lot of problems start during Construction Estimating. If the numbers aren’t right from the start, it snowballs into budget overruns, delays, and unhappy clients. What about you? What’s the biggest mistake you’ve seen happen on a job? Was it bad planning, bad communication, or something else entirely?