Taken from https://bitcoin.org/en/alert/2017-07-12-potential-split
Last updated: Thu Jul 13 2017 05:21:00 GMT-0600 (Mountain Daylight Time). This page will be updated when new information becomes available. See the list of updates.
Chain split
Since Bitcoin’s inception, its network has facilitated hundreds of millions of transactions. As a result, different groups of people (developers, investors, entrepreneurs, etc.), have debated on the best ways Bitcoin can be optimized to allow it to exponentially scale even further. In recent months, alternative software has been released that represents some of the interests of the aforementioned groups of people - software that is scheduled to go live toward the end of July.
The Bitcoin block chain, which is a record of all Bitcoin transactions to date, relies on a network of thousands of Bitcoin nodes running Bitcoin software. On Mon Jul 31 2017 18:00:00 GMT-0600 (Mountain Daylight Time), the Bitcoin block chain may experience what is known as a chain split. This is when a portion of Bitcoin’s nodes run software that another portion of nodes are not fully compatible with. As a result, some nodes may propagate confirmed transactions that other nodes may not accept or recognize. This may result in unreliable confirmation scoring for an unknown length of time. This means that any bitcoins you receive during this period could disappear from your wallet or be a type of bitcoin that other people will not accept as payment, until the situation is resolved.
Once the situation is resolved, confirmation scoring will either automatically return to their normal reliability or there will be two (or more) competing versions of Bitcoin as a result of a split block chain. In the former case, you may return to using Bitcoin normally; in the latter case, you will need to take extra steps in order to begin safely receiving bitcoins again.
This post currently describes what actions you can take to prepare for this situation. Subsequent to Mon Jul 31 2017 18:00:00 GMT-0600 (Mountain Daylight Time), we will update this post as best we can with relevant information, but you are also advised to monitor other Bitcoin news sites and community resources for updates and to cross-check all information, as someone may attempt to spread false news in order to exploit the situation.
Remember that you alone are responsible for the safety of your bitcoins, and that if you lose control of them for any reason, there is nothing the operators or contributors to this website can do to help you.
Note: there is a chance a milder level of disruption could start between now and Mon Jul 31 2017 18:00:00 GMT-0600 (Mountain Daylight Time). If that is the case, this post will be updated with details.
Preparation
If you accept bitcoins as payments, we recommend that you stop accepting Bitcoin payments at least 12 hours before Mon Jul 31 2017 18:00:00 GMT-0600 (Mountain Daylight Time), although 24 to 48 hours earlier may be safer. This will give time for all pending payments to confirm on the Bitcoin block chain before the event.
If you send bitcoins as payments, note that many services may stop accepting bitcoins at Mon Jul 31 2017 18:00:00 GMT-0600 (Mountain Daylight Time) or earlier.
Be wary of storing your bitcoins on an exchange or any service that doesn’t allow you to make a local backup copy of your private keys. If they accept transactions during the event, they could lose money and will likely spread those losses across all their users. If there end up being two or more competing versions of Bitcoin, then they may refuse to give you your bitcoins on versions they don’t like.
Bitcoin may experience significant price fluctuations in relation to other currencies. Learn more about price volatility and ensure you aren’t holding more bitcoin than you can afford to lose.
During the event
Do not trust any payments you receive after Mon Jul 31 2017 18:00:00 GMT-0600 (Mountain Daylight Time) until the situation is resolved. No matter how many confirmations the new payment says it has, it can disappear from your wallet at any point up until the situation is resolved.
Try not to send any payments. During the event there may be two or more different types of bitcoin and you may send all of the different types to a recipient who only expects one type. This would benefit the recipient at your expense.
Be wary of offers to allow you to invest in the outcome of the event by “splitting” your coins. Some of these offers may be scams, and software claiming to split your coins can also steal them.
After the event
We will update this section with more information after Mon Jul 31 2017 18:00:00 GMT-0600 (Mountain Daylight Time). Please monitor this page accordingly and wait until multiple news sources that you trust have stated that the event is resolved before returning to normal Bitcoin use.
Document history
Note: The information contained herein is not to be construed as an official statement by Bitcoin Core. Bitcoin.org and Bitcoin Core open source projects are run by separate teams.
A full history of this document is available. The following points summarize major changes, with the most recent changes being listed first.
-Thu Jul 13 2017 05:21:00 GMT-0600 (Mountain Daylight Time): add general info about split.
-Wed Jul 12 2017 06:00:00 GMT-0600 (Mountain Daylight Time): initial version.