File changing in snapshot

["Oliver O." <oliver.o456i@gmail.com>]

I seem to have observed a file on a (writable) snapshot changing 
although there were no writes occuring on the snapshot itself. This is 
not supposed to happen, right?

Sequence of events:

1. A (writable) snapshot @home-2014-04-16 is taken on a @home subvolume 
mounted at /home.

2. The current subvolume (@home) is used (via /home) for continued write 
access, no writes are supposed to occur on the snapshot.

3. The snapshot @home-2014-04-16 is backed up (using rdiff-backup) to a 
different disk.

4. As the backup is compared to the snapshot @home-2014-04-16, one file 
differs.

Further analysis:

The file in @home-2014-04-16 changed its contents (but not mtime or 
other metadata) after being backup up from that snapshot. 5 bytes differ.

The previous snapshot @home-2014-04-10 contains the file in its original 
form, which is identical to the backup just taken from @home-2014-04-16.

The file change appears identically
- on the last snapshot @home-2014-04-16 and
- on the current subvolume used for writing (@home).

btrfs scrub reports 0 errors.

At the time the snapshot @home-2014-04-16 was taken, the file (an Excel 
file) was probably accessed through Samba, which supposedly uses kernel 
oplocks (if that makes a difference).

Versions:
- Kernel 3.11.0-19-generic (Ubuntu 13.10)
- btrfs-tools 0.19+20130705-1
- Samba 3.6.18

Any ideas?