Dogecoin has forked and the issue is still being resolved. There are currently two competing chains, one being put out by older clients (version ~1.4) and one being put out by newer clients (version 1.5+). According to observant redditors, the fork appeared at block 104678.
What this means is that there are currently two competing Dogecoin blockchains. Eventually, one of them will "win" and only transactions recorded on that blockchain during the duration of the incident will actually go through. It is currently advised that you minimize your transactions during this time to help things blow over sooner.
As of now, the blockchain from clients version 1.5+ is longest and least likely to be orphaned.
What do I do?
Update your client.
Has anything like this ever happened before?
This is almost an identical happenstance to the Bitcoin fork of March 11th, 2013 when a fork appeared caused by Bitcoiners, and mining pools especially, being on 0.7 and 0.8 versions of the Bitcoin client. Bitcoiners were advised to switch back to 0.7 as that turned out to be the correct (longest) chain. The Bitcoin community as a whole survived that ordeal and mainstream media even picked up on it. What could have been a disastrous event for the Bitcoin network as a whole was relegated to a mere blip in Bitcoin history.
However, unlike the March incident on the Bitcoin blockchain, the blockchain has decided that the longer chain created by the newer software is in fact the correct one. Instead of downgrading clients, all Dogecoiners at this point should be updating to the newest Dogecoin 1.5.1 client.
As with the Bitcoin fork of last year, the main individuals that absolutely need to be changing their version of the client are mining pool operators. The Dogecoin subreddit, which is one of the most active Cryptocoin themed subreddits, beating out even /r/Bitcoin at times, has mobilized in fantastic fashion. Through prominent posts, it is easy for even the youngest shibe to check which pools are on the correct version. If you are merely a Dogecoin digger you should point your miners towards a listed pool to resolve this issue as fast as possible.
It bears repeating over and over again, this is NOT a 51% attack and the network security of Dogecoin and all other Bitcoin-derived Cryptocoins is just as strong today as it was yesterday.
However, since there are technically two chains propagating at this point, any merchant or user that is on the incorrect fork and unaware of the fork, could potentially think a transaction is completed when in fact it is not.
Back in the March Bitcoin fork, the days after the fork revealed a "successful doublespend" against OKPay. In fact, OKPay's system credited a user for their Bitcoin deposit based on 6+ confirmed transaction on the blockchain; however, the confirmations were on the forked (soon-to-be-orphaned) chain and when the entire network fixed itself and resumed mining on the proper blockchain, said transaction disappeared. The user was left with 61 BTC both in his wallet and in his OKPay account. This incident was resolved between OKPay and the user in a timely manner (he returned the extra BTC).
This type of "double-spend" is only possible with a less-than-vigilant centralized point of failure that continues to process transactions as normal during a fork. Needless to say, OKPay and most security-conscious Bitcoin-handling firms are a lot more careful to look out for fork situations. It is possible that similar events will be revealed over the next few days in the Dogecoin economy.
Remember: don't panic.
Even if silly double spends are revealed later on due to this fork, as a whole the Dogecoin community will be stronger for having stared down this 4-pronged attack by the forces of chaos.
Don't ever forget just how secure Bitcoin-based systems are.