Usually when this type of error happens, it is indicative of hardware failure. Run some hardware diagnostics on your computer and check for any hardware failures, particularly in the hard drive and in the RAM.

This topic has been moved to Trashcan.

https://bitcointalk.org/index.php?topic=1912976.0

Basically a duplicate.

Start Core with -zapwallettxes and that will clear out all unconfirmed transactions from your wallet. Then you can spend the coins again.

You need to have the private key to an address in order to spend the Bitcoin associated with it. A watch-only address is an address in your wallet that your wallet does not have the private key for. You cannot spend from a watch-only address in the wallet where it is watch-only, you must spend it from the wallet that actually has the private key.

Unless you can figure out where that watch-only address came from, the Bitcoin is lost.

Script versioning is required in order to redefine opcodes or just even add new ones. Added new opcodes cannot be done through a soft fork (except by redefining the OP_NOPX opcodes) since old software will not know what an opcode is and fail to verify the program. With script versioning, the old software can just ignore newly versioned scripts and assume that they are true so new opcodes and opcode redefinitions can be done through soft forks.

Your question is very hard to understand. Your post is off topic for this thread. Make a post in the Technical Support forum and include the transaction ids involved.

You would have to modify Bitcoin Core to accept double spends since Bitcoin Core will not allow double spends into its mempool for obvious reasons. Block explorers, in the interest of showing as much information as possible, run different software that allows double spends so that they can show them.

IIRC BitcoinXT allows double spend accepting and relaying. It was part of the original feature set of XT before it implemented BIP 101.

Cryptocurrencies are not like companies. A coin cannot be "bought out" or "merged" with another coin.

Your wallet does not need to be online to receive Bitcoin.

Technically, there is no such thing as a "Bitcoin" that is sent to your computer directly. What happens is that a transaction is created, and in that transaction is an output that you can spend. Said transaction is broadcast to everyone on the network and is included in the blockchain once the transaction has a confirmation. When your wallet next goes online, it will scan the blockchain for all transactions that contain outputs it can spend and for transactions that have spent from outputs that it can spend and then update accordingly.

It is not recommended for you to solely use that one address for receiving all of your payments. The recommended practice is that you should use a new address for every new payment that you receive, but sometimes that is not possible. However you should post your address for a couple of reasons. First it acts as a proof of your identity; if you can sign a message with that address, you can prove that you are yourself as you hold the private key to that address. Secondly, if you accept donations or tips, having a posted public address is useful as people can just send you money without you giving them an address.

For all payments other than unsolicited donations (i.e. payments where you are in contact with the person paying you), you should use a new address for each payment as that is better for security and privacy.

Download Mycelium onto your phone. Use that to import the private key of your paper wallet into Mycelium so that you can spend the funds. It can scan the QR code for the private key, and if you encrypted it, it can decrypt it too provided you know the password.

Yes. That is exactly what happened.

No. That "extra spend" that you see is called change. The Bitcoin is yours and is being sent back to your wallet.

 Bitcoin works by creating and spending transaction outputs. When you receive Bitcoin, you get a transaction output which you can spend from. When you spend from that output, it must be spent from fully. Thus when you spend from that output, some of it goes to whoever you wanted the Bitcoin to go to, the rest returns to you as change. You still control that Bitcoin and your wallet handles everything in the background, which is why you don't see the change anywhere in the GUI.

Change for Bitcoin is much like change with fiat money. For example, you have a $20 bill and buy something that only costs $8. You will receive $12 as change. The same happens with Bitcoin, except that the "bills" are outputs and can have variable value.

Should be fixed now.

Read the section on "Asking miners for help" on this thread: https://bitcointalk.org/index.php?topic=1802212.0

That is impossible to know. Again, given the complexity of the web of unconfirmed transactions, it could be an extremely long time, from several days to a few weeks. It is also possible that the transaction will never confirm.

All you can do is ask miners to confirm all of the unconfirmed transactions that your transaction depends on. However, given the complexity of the web of unconfirmed transactions, I don't think many miners will be willing to help you.

The problem is that your transaction fee is too low. Read https://bitcointalk.org/index.php?topic=1802212.0 for what you can do about it.

Since you are using blockchain.info's web wallet, there is not much that you can do.

No. It is recommended that you use a wallet with dynamic fees. Do not use a fixed fee or fixed fee rate.

He probably submitted it to ViaBTC's transaction accelerator. Submitting it there (or to any other transaction accelerator service) does not guarantee that your transaction will confirm within a given amount of time. It just means that the transaction has a higher chance of being confirmed.

Your transaction spends from two unconfirmed transactions. Those two unconfirmed transactions spend from a number of other unconfirmed transactions. Your transaction cannot confirm until all of the unconfirmed transactions which it depends upon (the ones it spends from, the ones that those spend from, and so on and so forth) are also confirmed. Since you have a huge mess of unconfirmed transactions that your transaction depends on, it may be a very long time before your transaction confirms. It may be possible that your transaction will never confirm if any of the several unconfirmed transactions ever gets double spent and the double spend confirms.