We were getting an error about a missing DLL, we get this about once per year on OS boot. The DLL is always there and does not move, but workflow does not load it, so we restart the workflow service and then it loads the DLL.

Any workflows that use that DLL will throw an invocation error when trying to start them and any of those workflows that were invoked, will not throw any activity error but will never start. The invoke will just sit in blue and the workflow status will just be active. It is as if the invoke is still waiting on the workflow to finish, but a search shows that the workflows are not running.