Purpose limitation, data minimisation and accuracy

Purpose limitation

Collect data for specified, explicit and legitimate purposes, and don't reuse it for a new purpose that's incompatible with the original one. If you collected an email address to send a receipt, you generally can't start using it for marketing without a proper basis for that new purpose.

Data minimisation

Collect only what is adequate, relevant, and limited to what is necessary. If a form asks for a date of birth but the service has no age-related requirement, that field probably shouldn't be there.

Accuracy

Keep personal data accurate and, where necessary, up to date. You must take reasonable steps to correct or erase inaccurate data without delay.