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When looking through the users pages, there appears to be users with negative reputation on the last page.
But, when the page is visited, the users have fairly large amounts of reputation. 
Is this some sort of short-overflow bug, or a joke maybe?
EDIT: Someone stated that negative reputation would be cruel here: Why does reputation start at 1, and have a lower bound of 1?
You're looking at two different periods of time.
In the first view, you're most likely looking at a week or month timespan. The second view shows the reputation of a single user the entire time since they joined the site.
The users page, by default, is showing users who lost rep, and how much, during the last month. You can change the timespan using the filter on the page (see the end of this answer).
On their profile page you see their total reputation since registration.
See the filter on the right:
