Tried to find if this bug is already reported, but I failed. So here it goes:
On all my machines, the Webmin System Information page sometimes shows that there are software package updates. When I click on the link or go to the appropriate module, it shows there are no updates. Refreshing package lists does not help this. There are actual updates, but these can't be installed through webmin, must be done through ssh or other means.
Systems running Ubuntu 10.04, up to date. Browser is Firefox 3.6.28, but doubt this matters.