That title sucks, but it's the best way I can briefly describe the issue.
OK, so say you start off on page 1... Nothing you want to read, move on to auto-loading page 2. Now you click on something, read it, then go back (using normal back button) and... you're actually on page 3, not page 2. What seems to be happening is the browser autoscrolls you to the point you left from which causes pinside to load additional page of posts to make up the difference.
In a more extreme example, say you start on page 2, then scroll until you read something on page 4. When you hit back, you wind up in the middle of page 6 (the difference from page 4 as you were originally from page 2).
This occurs on win7-64/Pale Moon 64 25.5 (should be latest), but has been happening I believe since the original mobile rollout. I assume since it's under Pale Moon, Firefox would be similarly affected.
It does not seem to happen on chrome, but it was a very quick test I just did.