Written answer received on 24 June 2011:
When a major event takes place in London, London Underground staff are in fact generally instructed to keep gate lines closed. There may be periods when the gate line at a particular station, say Westminster, has to be opened for safety reasons due to the sheer volume of passengers. However, passengers will normally still be able to touch in or out and are reminded to do so.
Auto-completion is used to assist customers who have not touched out when gates have had to be opened. When such customers next touch in, at Westminster or an adjoining station, auto-completion overrides the maximum fare that would otherwise apply to their previous journey.
Auto-completion was applied successfully at Zone 1 stations on the date of the royal wedding, with over 10,000 earlier journeys made complete when the customers subsequently touched in.
Auto-completion is a setting that is applied to gates and is not a 'fare' or 'charge'. TfL does not raise any revenue from the adoption of this setting.
In addition to 'auto complete' any customers who do not touch in or out can obtain a refund from a ticket office or the helpline.
I have asked Transport for London to meet with you, if you wish to be further briefed to clarify the issue.