One of the HTMLib topics that you read has not been recognised. This means that it won't have an entry in the history list. I'd be very grateful if you could isolate the particular topic (it's still navigable to by using the 'Back' button in the HTMLib) and let me know that it isn't showing up in the History List.
It could that you attempted to send internet mail using one of the HTMLib external mail links and the mail could not be sent - this causes an entry in the History list. Navigating back with the 'Back' button should highlight whether this was the case or not.

Details can be mailed to me at cmlehunt@swan.ac.uk

Thanks and sorry for the inconvenience.