Free space inconsistency with df / du

[Jonathan Schleifer <js-netbsd-users%webkeks.org@localhost>]

Hi.

df -h shows me that I have 3 GB used, while du -sh / shows me it's 2
GB:

webkeks# df -h
Filesystem    Size      Used     Avail Capacity  Mounted on
/dev/wd0a     7.5G      3.0G      4.2G    41%    /
kernfs        1.0K      1.0K        0B   100%    /kern
webkeks# du -sh /
2.0G    /

Could anybody explain that to me? Does it mean I have file system
errors and that the number of free blocks is wrong or something like
this? Should I shut down that machine and do fsck? I want to avoid
shutting it down if possible because it's a server (same for
remounting / ro and doing fsck).

TIA,

-- 
Jonathan