Exactly what it says on the tin - any time I make a change in a SharePoint 2010 solution which results in web.config being updated by visual studio, it makes changes to the element
<machineKey validationKey="{key}" ...
Prevents login. Must be the key I specify.
Help!
To prevent the machine key being overwritten, make the following change:
Navigate to http:///Lists/HealthRules/AllItems.aspx
(You can confirm its this setting by using the "Run Now" "Feature")