Social desirability bias
The tendency to answer the way that reflects well on you rather than the way that is true.
It shows up wherever an honest answer is embarrassing: reported exercise, reading, voting, drinking, charitable giving, and every question about whether somebody read the terms. It is not lying so much as rounding towards the person the respondent would like to be, and it is stronger when the survey feels observed.
Anonymity is the main defence, and it has to be visible rather than promised: no login, no email captured, no name field. If your survey collects an identifier and then asks something sensitive, the respondent is right to notice.
Wording helps too. Asking about a specific recent period (“in the last seven days”) beats asking about habits, and offering a normalising preamble (“many people find it hard to”) lowers the cost of an honest answer.
Where it goes wrong
Sensitive questions placed after a name or email field. The order teaches respondents what the survey knows about them.