The .ost is in use and cannot be Accessed Error

I think this was the first time I came across this error. The error message happened when a user was trying to send a PDF attachment. Apparently Outlook locked the .ost file from some other process. The message was The email username.ost is in use and cannot be accessed.

Easily enough just go into task manager and End Task on anything that says Microsoft Outlook. Simple enough yet dumb enough error message.