Quote from: ThisManIsTesting on January 10, 2018, 03:45:36 PM

Hello Everyone,
I want to know what is Double spent in bitcoin, and if you can sign a transaction while you don't have sufficient balance (in this case the transaction won't be confirmed ?)

REAL "double spend" is a situation where two (or more people) are paid with the exact same cryptocurrency funds, and it is not possible to determine with certainty which person was paid first (or which person gets to keep their payment. Each participant has now way of knowing if their payment is any good or not, and the entire system has no way of resolving the situation.

REAL "double spend" is not possible in Bitcoin. The reason that Bitcoin became popular and useful is because it is the first decentralized solution to the double spend problem.

Bitcoin uses the "confirmation" process to allow the entire system and all participants to come to a consensus about the ordering of transactions so that everyone can know with certainty whether any transaction is valid or not.

However, Bitcoin (and many other cryptocurrencies) have a time delay between after the transaction is created and shared before it is confirmed. During this time, it is possible for there to be multiple transactions that are all trying to spend the exact same cryptocurrency funds. The COMMON USE of the terms "double spend" within decentralized cryptocurrency communities is to refer to these competing unconfirmed transactions. Only one of them will ever confirm, and then the others will become invalid and will be rejected by the system, but during the time that the transaction is waiting to be confirmed it is possible to have multiple "double spend" unconfirmed transactions.

You can sign any transaction you want. But, if the transaction is not valid, then it will be rejected by ALL participants in the Bitcoin system.