The Turing Test and the Captcha are ways that humans differentiate themselves from machines.
Computers get messed up when things are not as predictable as the specific standardized rules or procedures that they are programmed to understand.
The more unique and stylized ones expression, in this case simply the way one personally writes letters and numbers, the more easily computers fail to interpret data accurately.
Machines, computers and robots, require a simplified and standardized set of parameters or rules to operate within, unlike the less bounded limits of human creative sentience.
We should know whether or not we are actively or passively engaged, and whether or not our programming is our own, otherwise we are simply living machines.