I was helping to one of my customers where his Sharepoint web sites stopped working unexpectedly. All of the web sites were failing with HTTP 500 Internal Server Error message. When I checked the details of the error message, I have found that the issue was related with the bitness of the application pools: all pools were running as 32bit. Changing the application pools bitness to 64bit solved the issue.
The question was who changed all of the application pools' bitness from 64bit to 32bit. After investigating the issue I have found that the customer has changed the bitness in Application Pool Defaults setting. This change applied all of the application pools immediaetly.
I have tested that behavior on my machine and found another interesting behaviour: some of the changes applied to some of the application pools. I will investigate this one and update the article as necessary.
Long story short, please make sure that you are taking a backup of applicationHost.config file before making a change or use change history backup files to revert the changes.