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?
Yes I have the exact same scenario. Thought this was a bug. However the user running the workflow had a custom permission of contribute with no delete. This was fine until the workflow went to delete the unwanted tasks from the task lists. So I've changed the permission back to 'contribute'.
I would check what Joanne suggested, especially if it is the same users that have the issue responding.
Other than a problem updating the task, I'm not sure about timing issues here (based on the internals of the action itself it should be fairly resiliant to these sorts of race conditions, you shouldn't see a task in the list before the workflow is already ready to be notified it is completed).
To get into it further it would be good to know: