As most of you will now there are very specific hard limits within SharePoint that may cause issues on your site if you ignore or go over the limits. Once of these limits is based around ACL's within SharePoint.
On a recent project I worked on I was getting the following errors within search when trying to index the content of the site.
After researching and trying the options I was not any further forward. However one of the suggestions (thanks Shane) was to look at clearing the "UserInfo" table within the database, yes I know this is not supported really; it was done in development to see if this would work. Once completed the search did index the content again and seemed to work. However we then has issues with user accounts not having access to content etc, so back the beginning again, but this did give us something to look at.
I decided then to look at the security groups and accounts that were being used by the site and between me, Shane and also Gary we found that there was a SharePoint site group that contained every FBA user from the back end LOB system, in excess of 300 thousand individual users added to the group. This was the root of the issue.
So the lesson here is, to check your system for things like this. Check with your vendors or any LOB systems that profess to have SharePoint integration especially authentication and ensure that this is not the approach they are taking. This issue alone took the production search capabilities off the site for over a week, so be aware of this issue.
Hope this helps someone.