I was using Chrome at the time. It seems the issue was related to one particular task but it was only after the event that I realised it was not happening with other tasks. I can't now remember which one was causing the problem. I can only think I've manually typed in a date and Chrome is remembering (and therefore prompting) the entry (i.e. as it does for form fields such as names, addresses, etc). When I find the task again I will post a screenshot.