user141: When people talk about possible changes to how Bitcoin works they sometimes say a particular change would require a hard fork. What does that mean? Can a hard fork cause problems?

user1171: Simply put, a so-called hard fork is a change of the Bitcoin protocol that is not backwards-compatible; i.e., older client versions would not accept blocks created by the updated client, considering them invalid. Obviously, this can create a blockchain fork when nodes running the new version create a separate blockchain incompatible with the older software. 
For potential future changes that would require a hard fork, see the associated wiki page.

141: <p>When people talk about possible changes to how Bitcoin works they sometimes say a particular change would require a hard fork. What does that mean? Can a hard fork cause problems?</p>
 1171: <p>Simply put, a so-called <strong>hard fork</strong> is a change of the Bitcoin protocol that is not backwards-compatible; i.e., older client versions would not accept blocks created by the updated client, considering them invalid. Obviously, this can create a blockchain fork when nodes running the new version create a separate blockchain incompatible with the older software. </p>

<p>For potential future changes that would require a hard fork, see the associated <a href="https://en.bitcoin.it/wiki/Hardfork_Wishlist">wiki page</a>.</p>
