Inconsistent information in your database
The blog post “Rethinking Form Validation” describes an interesting idea (apparently inspired by one of Alan Cooper’s books): While developers are fond of only storing information that is fully validated, it may help end users if they can store inconsistent data. Related examples include forms that force you to only enter digits for phone numbers (no spaces, dashes, parentheses, etc.) or some obvious characters being forbidden from passwords. Validation should be unobtrusive, because there are always going to be unforeseen cases where rigid control works against the user. Eclipse’s handling of Java syntax errors is exemplary: You are warned about them, but you are not prevented from continuing your work.