And that is why, if you're the editor, you have to have a zero-tolerance policy.
Since I've been an editor for 30 years, that's really the only viewpoint I know. I can see as a reader that it might be fun, or funny, to allow inside jokes and the like. But as an editor, you just can't let it happen. You can't open that door, even a smidge.
As soon as you let, say, Neal Adams put in something not approved through editorial process, then it becomes a judgment call. Why can't Adrian Syaf put something in, when Neal Adams can? Why, because what Syaf did is "bad," whereas what Adams did was "funny."
Who says? Well, I'll tell you: a judge. Because if you let Adams do it, then Syaf can do it. If you say Syaf can't but a white guy can (with a wink and a chuckle all 'round), then you've got a lawsuit. And it's a valid one that you will probably lose.
If you're a professional, you can't get sucked into the "subjective" game. Either you allow it, or you don't.
Here's a quick and dirty article about other cases where things slipped through: "Hidden messages in comic art are surprisingly common"
Here's a CBR piece about one thing that got through, not just in the original, but in reprints: "Marvel Fired an Artist Over Hidden Messages in a Comic – in 2001"