user22: I seem to recall, long ago, being told that difficulty couldn't change more than a certain amount at each retarget, but I can't seem to find what that factor is (or even confirmation that such a factor exists) no matter how hard I google. Does such a factor exist and if so what is its value?

user22: Well doesn't that figure, the moment I ask the question my google-fu kicks in.
This wiki article states:

A single retarget never changes the target by more than a factor of 4
  either way to prevent large changes in difficulty.


user659: The difficulty never changes by more than a factor of 4 (except for in a special case that only applies on testnet).
The code that enforces this is here.

22: <p>I seem to recall, long ago, being told that difficulty couldn't change more than a certain amount at each retarget, but I can't seem to find what that factor is (or even confirmation that such a factor exists) no matter how hard I google. Does such a factor exist and if so what is its value?</p>
 22: <p>Well doesn't that figure, the moment I ask the question my google-fu kicks in.</p>

<p><a href="https://en.bitcoin.it/wiki/Target">This wiki article</a> states:</p>

<blockquote>
  <p>A single retarget never changes the target by more than a factor of 4
  either way to prevent large changes in difficulty.</p>
</blockquote>
 659: <p>The difficulty never changes by more than a factor of 4 (except for in a special case that only applies on testnet).</p>

<p>The code that enforces this is <a href="https://github.com/bitcoin/bitcoin/blob/d62a1947be5350ed60066ccacc7aba43bbdf48fb/src/main.cpp#L875">here</a>.</p>
