This Coindesk article says that
So as transactions are being created, malformed/parallel transactions are also being created so as to create a fog of confusion over the entire network, which then affects almost every single implementation out there,.
Antonopoulos went on to say that Blockchain.infos implementation is not affected, but some exchanges have been affected their internal accounting systems are gradually going out of sync with the network.
I think I understand the first part; as transactions are created, hundreds of transactions are created that are identical except for a mutated signature are created in the network. The second part confuses me a bit, though. Do the exchange's accounting systems get out of sync because some of the original transactions aren't going through because a mutated version has already been confirmed in the blockchain? So for example, Mt. Gox, which was using an automated system to approve withdrawals (as stated in the article), would consider a withdrawal transaction approved before it was actually confirmed in the blockchain. Once a mutated version of that transaction was confirmed in the blockchain, Mt. Gox's accounting system had a problem because a transaction it assumed was confirmed (and therefore had already "lost" the Bitcoins for) wasn't actually confirmed.