In the older console game Ratchet & Crank on the hover board level the intro has an explanation cut scene. An alien woman acts as the trigger and interaction point for the dialog that tells the player what to expect in the coming level. Immediately after this the game expects the player to proceed to the starting point of the race.
However, if the player proceeds to make multiple horizontal flip jumps the bust of the alien gal starts to expand. This information has been out there for some time, but I was unable to confirm if this was an Easter Egg or glitch as some have suggested.
Many would agree that this is an Easter Egg as it has all the elements of one. However it is a very odd one and such a random event given the nature of the game. Also, the effect is subtle, progressive, has a maximum limit and is tied to a specific player maneuver. Together these could suggest a model / programing glitch.
The glitch theory is based on the fact that the female alien character is stuck in one spot and just her upper torso, head, and arms move and adjust to the player's position relative to her. Your character can stand or run, jump, flip and even fly. In all cases the gal will follow you so long as you are in front of her. If you move past (behind) her or too high she reverts to looking ahead. What this means is her upper body adjusts and moves so long as you are within a prescribed range, otherwise it resets.
What the horizontal flip does is possibly place the player right at the elevation where she would either move to follow or reset to neutral stance. Thus, it is possible that her torso model gets slightly elevated but never gets the reset value. Then, when the player repeats the move, her expansion is from that +1 state, in essence a +2 state. If one repeats, they continue to build upon that "follow the player" but without a reset. Only when the overall model reaches some maximum limit, possibly a hit box or motion limit that the chest expansion stops. It really could be a glitch as at the larger she gets it becomes obvious that it is a single control point that keeps shifting that cause the scale increase.
Not every jump causes the inflation by the way. Only when you just just right.
Anyway, an odd Easter Egg.