Illegal choice error in log file
Someone (or something) is trying to submit a value for field_item_type_tid that isn't in the list of allowed options for that field (as prepared by the form API prior to rendering the form/element).

The FAPI is objecting to that attempt (as it should; it's a security feature), so the user will receive the standard "Illegal choice" error message. 

It might be an attempt at a hack, it might be something weird instigated by a user/browser extension, or it might be the result of some javascript trying to change the allowed values on the client-side, without informing the backend that the options need to change.

Without analysing the page and all the involved parameters it's only speculation, though. It might be being caused by any of the above, or perhaps some other external factor.