Failure to pass on bad news to the client. Compounded by making stupid choices to avoid having to pass the bad news onto the client in hopes the bad news will go away.
For example, developers report there is a show-stopper bug, cannot deploy until it is fixed. It will take a week to fix and test. PM insists we deploy on schedule with bug unfixed because the client is insistent that the schedule be met. First day out, the bug is hit and the client is screaming because we didn't find and fix it before deploying. The pain gets worse when you avoid telling the client.
We often know a long time ahead that we won't meet the schedule. Developers often tell their managers who tell the PM that. PM fails to tell the client that until the deadline is already passed making things worse. Especially when it won't be missed by a few hours or a day but by several weeks.
Clients will remember deploying a buggy bad project far more than they will remember slipping the schedule.
Another one that gets me is the "we must fix the process attitude" that means every little tiny error no matter how unlikely it is to be repeated must have new process steps added to avoid ever having it in the future. Of course every new step added to the process adds to the time to complete work (which we did not add to the schedule) and adds new different possible errors. Just accept that human error will happen and no system can be made so that no errors will ever happen. Change the process when there are major flaws int it not every little minor issue that comes up.
Finally not pushing back to the client to insist on more time and a slip to the deadline when features are added or when the client causes the delay. If client must deliver something before you can do your work (hard to design a data import without the file in hand for instance), they need to be told that every day they slip their deadline to us, we slip ours the same amount of time.
Solution: Pass on the bad news as soon as you know about it, delaying will only make things worse. If you can't handle dealing with telling people bad news, you are in the wrong profession.