This site may help you find what you're looking for. Examples are:
"One classic wall-breaking moment in Metal Gear Solid is the advice to contact Meryl by codec by looking up her codec frequency on the back of a disc case, a reference to the game's disc case.
- Can't get Meryl's frequency because you threw away the jewel case or
lost it? No problem! Contact Colonel enough times and he will give it
to you! But, of course, you have to check the Frequency List for it.
- A similar moment appeared in its earlier prequel Metal Gear 2: Solid
Snake, where Snake is told to look up a radio frequency on a package,
a reference to the game's package."
"In Rayman 3: Hoodlum Havoc, Murfy straight-out takes out a copy of the game's manual and uses it to explain to Rayman what's going on."