Its worth checking your $1 TOA stamps (and the rest of course) for errors. This stamp does seem to have a few of these, primarily perforation errors. Those that I have found all seem to to have extra horizontal perforation lines. None are in the same place which rules out something in the design which confused the perforator, so it is just down to random manual error. So why where these not rejected? Perhaps they were intended to be found!

Here are a couple of printing errors, though I am not sure the cause of the first. Towards the top of the tower one of the blocks of stone appears to be yellow. Looking at whole sheets and other stamps shows no other examples so I am left concluding that it was something on the paper.
The second has a major shift of the yellow ink to the left, resulting in a rather odd coloured stamp.