When copying multiple files, if at some point one runs out of disk space, I get:

(thunar:7375): GLib-WARNING **: GError set over the top of a previous GError or uninitialized memory.
This indicates a bug in someone's code. You must ensure an error is NULL before it's set.
The overwriting error message was: Failed to copy "filea" to "fileb": Failed to write data to "fileb" (No space left on device)

(thunar:7375): GLib-WARNING **: GError set over the top of a previous GError or uninitialized memory.
This indicates a bug in someone's code. You must ensure an error is NULL before it's set.
The overwriting error message was: Failed to copy "filea2" to "fileb2": Failed to write data to "fileb2": (No space left on device)

and so on.

I use Thunar 0.9.3 (Xfce 4.4.3) on gentoo.

This indicates a bug in someone's code ;-)