EXECUTE AS returning wrong user

Question

I'm running into a weird situation. This code is running fine on all but one server.

set nocount on
SELECT SUSER_NAME(), USER_NAME();
execute as user = 'guest';
SELECT SUSER_NAME(), USER_NAME();
revert;
SELECT SUSER_NAME(), USER_NAME();

This returns my username for the first and third statements on all servers. On the problem servers it returns a fellow dev's logon information instead of guest. Something like DOMAIN\joeblow.

Is there a SQL Server setting I should be setting or some other issue I don't know of? We're running SQL Server 2008 R2.