1. A method comprising: determining whether a user, who utilizes a computing device to interact with a computerized service, is either an authorized user or an attacker; wherein the determining comprises: generating a temporary input/output interference that causes an anomaly between (A) input gestures that the user performs via an input unit of said computing device, and (B) output that is displayed on a display unit of said computing device as a result of the input gestures; wherein the temporary input/output interference is a binary-type interference defined to trigger one of two possible manual user responses, wherein the two possible manual user responses comprise: a first possible manual user response that is performed by a majority of a general population of users; and a second possible manual user response that is performed by a minority of the general population of users; based on a level of uniqueness in the general population of users, of a particular response-to-interference that is identified in input-unit interactions of said user, determining whether or not to re-use said temporary input/output interference in subsequent usage sessions of said user; wherein the method further comprises: presenting to a user of an electronic device a screen comprising content and an advertisement; injecting a temporary input/output aberration that causes an on-screen pointer, that is on route to click within said advertisement, to deviate from its regular route; tracking user interactions with an input unit of said electronic device in response to said temporary input/output aberration; determining whether said user performed manual correction operations that fix said temporary input/output aberration; in response to determining that said user performed manual correction operations that fixed said temporary input/output aberration, determining that a click of said user within said advertisement was performed by a genuine user and not by a click-fraud mechanism; in response to determining that said user performed manual correction operations that did not fix said temporary input/output aberration, determining that a click of said user within said advertisement was performed by a click-fraud mechanism.