I was doing a demo today around SharePoint Designer 2010 workflows and found a bug in the design. When you create a workflow and want to edit the xoml file for the workflow, you must check it out. Why would you do that you ask? Well, the Designer Interface is far from perfect and you may need to move a step around in case you didn't realize you started to sub-nest your steps. The only way to do this is to edit the .xoml file. In order to edit the .xoml file, you must "Check Out" the file first. After you check it out, you can then edit the file in Designer via the XML Editor.
Saving the file but not "Checking It Back in" will cause the file to lose resolution to the SharePoint workflow code. This will cause the following error in the log files:
System.ArgumentNullException: Array cannot be null. Parameter name: bytes at System.Text.Encoding.GetString(Byte bytes) at Microsoft.SharePoint.Workflow.SPNoCodeXomlCompiler.CompileBytes(Byte xomlBytes, Byte rulesBytes, Boolean doTestCompilation, String assemblyName, SPWeb web, Boolean forceNewAppDomain)
Checking the file back in will allow the workflow to execute again.
RESOLUTION: A check out should cause the workflow to be unpublished. You would then need to save and publish it again after check in