Can not complete the project even thought task are 100% complete

Did you check to see if you have open requests (issues) tied to the project?

We sometimes have cases in which users have added an adhoc Workflow to a task by accident needing the closing of that task to be approved.

Can also be cause by date conflicts (as we just found out): make sure the end date of a task is after the start date (and also not the exact same time).

Another cause is if a parent task was originally a child task and for whatever reason had time against it, assignements, or is maybe just gitchy. Recalculate usually fixes these.

Also make sure all parent tasks are aggregating properly: that all child tasks being 100% also causes the parent to be 100%.

We've been running into these more frequently lately. We don't know if these are new habits, or the system being pickier now about bad existing habits.