I tried to install an application in SalesForce called Inline Account Hierarchy, which allows you to make a collapsible tree in visual force for hierarchical objects, but I received the following error:
Your requested install failed. Please try this again.
None of the data or setup information in your Salesforce organization should have been affected by this error.
If this error persists, contact Salesforce Support through your normal channels and reference number:
Google led me to this post on Force's discussion boards, where user A_Smith (Andrew Smith) gave the following answer:
I looked on the backend and believe you might have hit a very corner case issue. Do you have a local class called "test" in your org? If so, delete or rename that one. Basically, the apex in this package thinks your local class is the system class called test. That is causing the package install to fail.
Sure enough, I had a class I named test, deleted it, and the install worked.
Thank you Mr. Smith.