One should be cautious about running superuser commands since they certainly can be dangerous to your system. But after some research, Guimesquita2's reply worked like a charm on Mac OS X 10.8.3. Alternatively, Repair Permissions in disk utility did many things, but did not fix this specific unwanted behavior for me.
Working in the Terminal app, two unix commands, sudo command and chown owner-user:owner-group directory are concatenated:
sudo is superuser do, and "requests" one-time limited superuser access to the command (here, chown).
Guimesquita2's example targets the folder ~/.Trash by default, i.e. trash belonging to the user logged in.
The example command line will not change anything elsewhere in the system (such as another user's .Trash hidden folder). It seems to be pretty darn safe.
As a bonus, it also let me (again) see the unemptied contents of Trash which I could not see before.