[theamk]

It is pretty common to "verify" workers: a fraction of questions (often 1% to 10%) is asked-before questions with known correct answers. If those are not answered correctly, the entire dataset from this person is ignored. Depending on the platform, they might get paid less as well.

This is designed to detect workers who either did not understand the instructions, or those who don't care about those and answer randomly. But this works against intentional sabotage as well.