If you floss regularly your gums won’t bleed, source: me. My gums never bleed at the dentist, but they used to when I sucked more at dental self-care.

For the X-ray example, think about it this way: the doctor is like a bartender. One drink isn’t going to hurt you. The bartender can definitely serve you one drink. But if the bartender had to drink a part of a drink every time they served someone, that would become pretty dangerous for them indeed.