Techrepublic has an article with a list of 10 “illegal” job interview questions and they all sound so very normal, harmless and like small-talk but apparently steering towards discrimination:
#1: Where were you born?
#2: What is your native language?
#3: Are you married?
#4: Do you have children?
#5: Do you plan to get pregnant?
#6: How old are you?
#7: Do you observe Yom Kippur?
#8: Do you have a disability or chronic illness?
#9: Are you in the National Guard?
#10: Do you smoke or use alcohol?
Most of those questions seem pretty irrelevant because it is likely that you would already have submitted a job application before an interview answering them.
I honestly don’t think you could call those questions “illegal”, certain jobs require certain qualifications and personal circumstances, so it would be important to ask.