I marked a project as completed and then found out it wasn't completed. How can I get the project back on the list?

I'll shed some light on the status for the projects in QuickBooks Online (QBO), ellenm-edmondsum.

We can only set the project status as Completed, Canceled, or In progress. In this case, we can edit the project status to set it as "In Progress" in the meantime if you think there will be unexpected expenses after completing it. This will also get the Project back on the list. 

Alternatively, you can add some notes if you don't want to change the status. Simply open a project, then click the Edit button.

From there, you would want to add something like "Added unexpected expenses" or similar. You can check out this article for more details about the Projects feature from this article: Set up and create projects in QuickBooks Online. 

Need to run reports for your projects or transactions? Our articles can help you build one. Check out the list of guides in our Reports articles. 

Please know that I'm only a few clicks away if you have any other concerns or issues about Projects. I'll be more than happy to answer them. You have a good one.