The Orion 5281 CPU (revision D0) used in the QNAP TS-409 has a bug that can lead to a lock-up of the whole system under some circumstances. Fortunately, this only happens with the old ABI of ARM and not with the new EABI.
If you only use EABI binaries (i.e. Debian's armel port) on your TS-409, this bug will have no impact at all. This bug only shows up with old ABI binaries, and only with certain old ABI binaries. Please note that the EABI kernel shipped with Debian has support for old ABI binaries. If you have users on your TS-409 that cannot be trusted fully, you may want to consider recompiling your kernel without compatibility for old ABI binaries.
This bug is known and documented in the 5281 errata but a software workaround doesn't seem possible. However, this bug should have no impact as long as you use the EABI port of Debian without any old ABI binaries.