I got this error in my log file. It was on a select element for an exposed View filter:
Illegal choice 24alert(0) in field_item_type_tid element.
Is it someone trying to hack my site?
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.
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.