Could not load file or assembly 'Microsoft.ReportViewer.Common' or one of its dependencies. - File is being used

Users to our production site is occasionally receive this error and we can't seem to figure out why. If the user gets this error, they can refresh the page and it works. Any ideas?

Could not load file or assembly 'Microsoft.ReportViewer.Common' or one of its dependencies. The process cannot access the file because it is being used by another process. (Exception from HRESULT: 0x80070020)

I've experienced this many times, it's usually a debugger or antivirus locking files the website uses. I suspect it appears to work because at refresh time the file(s) are no longer locked.

Run the site without remote debugging and temporarily disable your antivirus. This should narrow the problem down.

See this and a related answer.