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Check your statements.

You receive many statements per month. Do you check all of them? Chances are that you do not verify them regularly.
Best place to start is to check your credit card statements , you will once in a while notice that you have been charged twice for the same invoice, or have been charged a bill that does not belong to you.
If you don't verify, nobody will tell you.

Buying stuff on the Internet.

You buy stuff on the Internet and when it is time to acknowledge the transaction, you click.
It takes a while before processing your card and that the online payment is completed.
You are a little impatient and click again, thinking that the first click did not work.
Surprise, when you receive your credit card statement, you note that you where charged twice for the same online payment.
If you had not checked your statement, nobody would have told you.

Sometimes, there are glitch on computers. It will not necessarily be the fault of the merchant. Most merchants are honest and they will refund your credit card right away.

Keep copies.

Keep all copies of the transactions and online payments you make with your card, as well as invoices. If you ever have a problem, it will be much easier to solve if you have all the documents handy.

Keeping your business or personal transaction copies will make it very easy to verify your statement. Many people just trash copies of their credit card transaction and invoices as soon as they have it in their hands. That is not a very good practice.

Warranties.

If you do that and have bought an article that carries a warranty for a year as an example and that the good in question has a problem after nine month.
What will you do to prove that the warranty is still valid if you do not have your transaction copy, online payment document and invoice?
Sure you can go back to your statement, try to have a copy of the invoice from the merchant and then submit the invoice to the manufacturer.
It may work but with how much trouble?

Extended Warranty

Further more, if you paid with a credit card that carries an extended warranty covering when you use it and that you do not have your transaction copy, how will the Card Company know that it is the good you bought with it that carries the extended warranty? Then for sure you are in trouble because the Card Company does not have records of the exact good that you bought.

Keeping your transaction copies and invoices is a must. Verify all your statements, bank accounts, debit cards , property taxes, electricity, heating fuel, water meter and any other.
You will save time and money.