What Could Cause The "Fatal Error - Headers Already Sent" Error?

I would like to know what would cause this error when you activated or deactiate a plugin:

Fatal Error.  Redirect could not be issued due to headers already being sent. Halting execution for security. Search http://docs.elgg.org/ for more information. (get this error upon installing and uninstalling this plugin)

Im not sure ad i have already searched elgg website and also the elgg docs but not seeing anything. If someone could direct me to this info i would really appreciate.