Sometimes hg st just don't work like expected... It shows something that I am sure I have put into .hgignore. This only happens on my Linux/Debian boxes while there's no problem with any of my Windows boxes. Weird? That's what I felt.
After encountering it several times, I found it's inotify's fault. inotify extension was enabled somehow on my Debian box, which brings up a long living process for each repository. It is claimed that for large repository, the extension can help you to get better performance. Unfortunately it gives my mercurial ill-behaviors. Simply disabling it is the cure. Put:
[inotify] autostart = False
in your ~/.hgrc or .hg/hgrc. If you want to have it enabled in certain repository, go head to set the option back on in the specific .hg/hgrc.