Merry Christmas Glitch

Explanation

In the game there are 4 springs where Ammy can deposit yen in exchange for the secret Celestial Brush Techniques Power Slash 2 and 3, and Cherry Bomb 2 and 3. These techniques cost 60, 360, 120, and 300 thousand yen respectively. For each pool, Ammy is given 3 quantities of yen she can submit at a time: 1/12th the required total, 1/6th the required total, and 1/3rd the required total. Thus, the fastest way to just get the technique would be to select the maximum quantity 3 times for every spring.

It is possible, however, to submit more than the total required amount of yen. For example, you can submit the smallest amount, then the largest amount 3 times. If the total amount of yen given is greater than the total required for the brush technique, the spring will spit out yen after the cutscene where Ammy received the technique. It seems the developers intended to return the exact amount of extra yen given back to the player, but the calculation is incorrect. The amount returned is instead 65,530 minus the excess. This is called the Merry Christmas Glitch (AKA Refund Glitch), due to being discovered by Auride on December 25th, 2019.

For example, if one goes to the PS2 pool and gives 5k yen, then 20k yen three times, they will receive back 60,530 yen, for a net profit of 55,530 yen relative to not giving any excess. The same pattern of giving the minimum quantity followed by the maximum quantity 3 times yields the maximum profit for all springs. If the excess is greater than 65,530, the amount returned wraps back around to 65,530, though this is pointless as it would be long beyond the maximum excess that is still profitable.

Demonstration: https://youtu.be/c6XmC5DsI7Y

This glitch is believed to be caused by a type or sign error. 65,530 is close to 65,536, which is 2^16. It's possible the returned yen is internally represented as a 2-byte unsigned integer (i.e. a short int), and the value assigned to it is the negative excess, when it should be positive. The value is then floored to the nearest 10 yen, for unknown reasons.

This is used in both NG All Brushes and Top Dog, where it significantly contributes to the yen route. Merry Christmas Glitch is not done specifically for the PS3 pool, as the profit is not great enough to justify the extra time taken.

Technique	Cost	Minimum Excess	Amount Returned	Effective Profit
Power Slash 2	60,000	5,000	60,530	55,530
Cherry Bomb 2	120,000	10,000	55,530	45,530
Cherry Bomb 3	300,000	25,000	40,530	15,530
Power Slash 3	360,000	30,000	35,530	5,530