Pathfinder's four degrees of success system is so good here. A crit is any time you beat the DC by 10. A crit fail is any time you get 10 less than the DC. The system itself then tells you on a bunch of different spells/actions/effects what happens on a crit/success/fail/crit fail. It's handy because often this means a regular fail can still mean something good happens for you. In the case of lockpicking:

Critical Success You unlock the lock, or you achieve two successes toward opening a lock that requires more than one success. You leave no trace of your tampering.
Success You open the lock, or you achieve one success toward opening a lock that requires more than one success. You leave behind damage that indicates the lock was picked on close scrutiny.
Critical Failure You break your toolkit and leave behind obvious damage. Fixing a broken toolkit requires using Crafting to Repair it or else swapping in replacement picks (costing 3 sp, or 3 gp for an infiltrator thieves' toolkit).