Unprocessed tokens appearing in URL alias

I've got a node type with the following pathauto.

product/[title-raw]-[nid]

Most of the time, this seems to work, but somehow, for some nodes, the path becomes corrupted and takes the form:

product/my-product-title-[nid]

with "[nid]" complete with square brackets, appearing in the url. In all instances of this corruption, somehow the "Automatic URL" checkbox is unchecked when I go back to edit the node. Reverting back to a previous version fixes it as well. Users are not unchecking the "Automatic URL" checkbox, nor typing [nid] into the alias textarea.

I suspect what's going on here is that something is getting corrupted during a node_save operation (that isn't saving a node-revision), but being unable to reproduce this I'm not sure where to start. I'll try and build some traps to catch when this happens.

Does anyone have a clue as to what I should be looking for? What process would cause the URL to contain the raw token?