Hi Meta,
It depends on the model of a router, function of a router, image on the router, configuration of a router.....
The below mentioned link has system and error messages for multiple platform and release.... you can go one by one and have the information that you need. Hope this helps...
Likewise you have these kind of links for many platforms...
http://www.cisco.com/c/en/us/support/switches/catalyst-6500-series-switches/products-system-message-guides-list.html
Regards
Karthik