Be careful of scheduled workflows. I had a nightly scheduled workflow that came back with an error. I knew there was no reason for it to error because it had been running for months with no problems. I contacted the server admins to check into the error on the server for me. They didn't see anything suspicious. I had the workflow running at a specific time. They told me that could cause a problem. I didn't know it but they ran patches and updates at the same time my workflow started. They recommended I ran the workflow outside of that window. This particular workflow had to run midnight or later but before employees arrived at work. I didn't want any employees updating the list while this workflow was running. I changed the time their updates were over. I haven't had any problems since. I also learned there were other workflows scheduled and the times they were running. I found that the workflows were put in a queue so your workflow wasn't guaranteed to run exactly when you wanted it to run. Sometimes my workflow started when I scheduled it and sometimes it would be an hour or later.
When you are creating a scheduled workflow, be sure to know the times other workflows are scheduled and any maintenance on the servers are to be performed. This will save you a headache later.
Lisa