Mistakes that I see often:

1. not keeping a running list of Risks and not being able to manage those risks effectively
2. always saying yes and never controlling change which leads to scope creep
3. not creating small clear milestones (this is why Agile and SCRUM use sprints, to force these milestones to be created!)
4. no proper monitoring of a project; no idea if 100% or 50% or 0% of a milestone is completed, no idea who has knowledge of which area of a project, no idea what stakeholders are expecting
5. no closing of a project; there are some tasks that have to be done at the close of a project and they're usually just not kept track of at all. Handing things off to another team means you have to document things for them.
6. not using time buffers to allow people to work on some necessary tasks for a limited time (think bug fixing for software with a minimum of 2 days and a potential maximum of 30 days in the schedule).
7. not building holidays/training into schedules, hard to work on a project when you expect 100% of resources to be available!

Most of these mistakes stem from the fact that there has been no formal education in project management techniques and tools and possibly no interest in learning those.