I don't think the 403 is really too funny or flavorful, maybe just get rid of the joke? Elsewhere, a button that says "Rats!" is hilarious to me, but "Peace Out" doesn't capture the same spirit or really make sense in response to hitting a 403 (you wouldn't say "Peace Out" in an analogous physical situation). But I think the button should be labeled "Go Home" or "Done" or "Cancel", not "Back", because it is very unlikely to take you back (that is, I think we should only label a button "Back" if it logically takes you backwards in a workflow, and has roughly the same effect as the browser back button).

For the 404, did a user actually find this confusing? Some mirth on 404 pages seems pretty standard across plenty of other sites (e.g., even Atlassian/JIRA have a bit of a visual joke), maybe we could just fix the "Focus" action there? I don't think the joke is exceptionally funny but I'd like to imagine putting whimsical art there some day or something, without needing to serious-business it.