I have a workflow with the "Start A Task Process" part. I've configured it to use Parallel Task Assignments, with "Wait for First Response." The assignment goes to two people. 95% of the time it works great. The workflow notes the first of the two users to respond and proceeds. I've noticed however, that 5% of the time when the first person approves (task complete), the workflow doesn't proceed. It gets stuck until the second person approves. My guess is that the first person approved too fast because there's no workflow error. I can't prove this however, because when the second person approves, SharePoint deletes the first person's approval task.
Has anyone seen anything similar?