The text book says that all money is fungible i.e. any one dollar bill is as good as any other one dollar bill. However, in practice, people do not believe in accordance with this principle. There are countless people in the US who have a credit card debt at 20%, though at the same time they save money for future. In reality, this is irrational. Here are common examples of mental accounts:

Classifying Money According To Risk:
Investors usually classify money according to the risk that they can take with it. There is a certain amount of money that they want to put in the bond market and another amount that they think they can speculate in equities with, and so on. In reality, these markets are cyclical. Such investors usually end up averaging their return, never really taking advantage of superior returns anytime.

Classifying Money According To Source:
Investors usually classify money as hard earned or the one earned through speculative activities. They do not mind losing money that was earned from trading itself. However, when it comes to losing money that has been earned through physical labor, they become very apprehensive.

Classifying Money According To Transaction Size:
Investors tend to ignore small transactions when they are doing mental accounts. This is where brokers and other intermediaries get rich at the expense of the entrepreneur. Great investors, such as Warren Buffet, have been mindful of the transaction sizes.