MtGox issued a statement that due to a “design issue” in Bitcoin protocol, they were having problems with withdrawing BTC and so they had to halt all withdrawals until the problem is fixed. https://www.mtgox.com/press_release_20140210.html

If you need a quick answer: there’s no bug in the Bitcoin itself. You may go to Bitstamp/Coinbase/BTC-E/Bitcoin-Central and buy more BTC with a huge discount before it gets back to $800-$900.

Long answer:

Unconfirmed Bitcoin transactions were always “malleable”, that is you can slightly change a transaction that “floats around” (not yet in the blockchain) and you wouldn’t break its signatures. You can’t change something important about it, like source transactions, amounts, order of inputs and outputs or other important metadata. What you can do is to add some bogus data or flip a sign on a signature that doesn’t change the meaning of the transaction, but changes its binary representation. (More info here: https://en.bitcoin.it/wiki/Transaction_Malleability)

What does it mean in practice? You may send a transaction ABC123, then someone may see it on the network, change slightly to ABC124 and send it too. If he gets lucky, ABC124 will be included first and ABC123 will never be included (because it’d be a double-spend). There’s no problem for the recipient of the transaction: they will still get all their money on the address they expect. But if they were watching the blockchain specifically for transaction ABC123, they will never find it there.

MtGox claims to be fooled this way:

User asks MtGox to withdraw some bitcoins to some address of the user’s choice. MtGox takes some of its own “unspent transaction outputs” and composes a transaction which sends funds to the user’s address. MtGox remembers a hash of that transaction (unique fingerprint of its contents) and begins to watch the blockchain for this hash to appear in it. User or someone else sees unconfirmed MtGox transaction in the p2p network. He changes some bytes in it to keep it valid, but make it different to change its hash. New, modified transaction gets included in the blockchain. MtGox has sent money where needed, but does not know about it. User also got the funds no problem - his personal wallet will show that he has the funds. Then, user goes to MtGox support and complains that the money did not go through. Or, MtGox themselves see that they’ve been watching for transaction for too long and could automatically re-send another transaction that sends some other “unspent tx outputs” to the same address (sort of, to “retry” the transaction). One way or another, it creates a lot of confusion for MtGox and initially may even lead them to sending the same money twice, or multiple times to the same user.

Is it a design issue in Bitcoin to allow slight changes in unconfirmed transactions? Yes, probably is. But it’s not entirely clear how it can be prevented at all. An immediate fix would disallow potentially useful more complex transactions and require a global network consensus to enforce new behavior. Zero-confirmation transactions were always known to be malleable and methods to limit their malleability were already discussed and deployed (e.g. transactions with non-canonical signatures may not be relayed by all nodes). But for all practical purposes, it’s a known feature, just like many other weird facets of Bitcoin. Those who build Bitcoin wallets, exchanges or payment processors must be aware of this and act accordingly.

MtGox had this problem because they didn’t know about this Bitcoin property. And usually transactions were not deliberately modified by anyone, so it was okay for the most of the time.

It’s not rocket science to fix the problem. For instance, MtGox may fix the problem this way: instead of watching blockchain for appearance of the specific hash of a specific transaction, they should instead watch if the address X (specified by user) got amount N (specified by user) from outputs Y, Z and W (owned by MtGox). This would guarantee that even if transaction is modified, they will see for sure if the users actually got the money sent to them, or not.