In any game where you are using anti-creeper makers and you are pooling the anti-creeper, once the anti-creeper hits maximum density (around 2100M) it will suddenly turn into standard creeper.
Note: This has only happened during custom maps where ore and anti-creeper emitters are very abundant.
Game Version: 0497
You may call this a design feature rather than a bug. The opposite is also true. Ultra-dense creeper turns into Anti-Creeper. The precise number is 2147M of either Creeper or anti-Creeper to invert.
You COULD call it a design feature, but only if it is actually intended for dense anti-creeper to suddenly convert to regular creeper. It's obviously a consequence of using a single signed 32 bit variable to hold the amount of creeper in any given location, and when the value overflows the bounds of the variable it results in a sign change. With code to handle the overflows, it could be made to cap out at the maximum value instead of continuing to increase (Or decrease) the value. Whether or not the existing behavior is a bug or a feature would depend on whether it is deliberate, or just an unintended consequence of the way the game state is represented in memory.
It's intentional that I don't prevent the over/under flow. I used to refer to it as Creeper 'fusion'.
It makes things interesting, and as a map maker i think it allows for more possibilities.