Repeat by:

1. Go to Special:Contributions for a user with more than 500 contributions (say, https://en.wikipedia.org/wiki/Special:Contributions/RoySmith)
2. Click the "oldest" link at the top, which gets you to https://en.wikipedia.org/w/index.php?title=Special:Contributions/RoySmith&dir=prev&target=RoySmith, showing the default 50 items.
3. Click the "500" link. This gets you to https://en.wikipedia.org/w/index.php?title=Special:Contributions/RoySmith&offset=20041221042420&limit=500&target=RoySmith, which still shows 50 items.

Expected result: you should be looking at the oldest 500 items.

If you traverse the path in the opposite way, i.e. clicking "500", then clicking "oldest", you get to https://en.wikipedia.org/w/index.php?title=Special:Contributions/RoySmith&dir=prev&limit=500&target=RoySmith which does indeed show the expected result.