update(id, attributes) – performs validation
update_all(update, conditions, options) – doesn’t validate, and reverses the operand order (what attributes to change first, followed by the records to change as specified by “conditions”, rather than the other way around). Note also that despite its name it doesn’t (necessarily) update all records.
update_counters(id, counters) – doesn’t validate, reverts operand order

instance methods:

update_attribute(name, value) – doesn’t validate
update_attributes(attributes) – does validate
update_attributes!(attributes) – does validate

A bit of consistency wouldn’t have gone astray. The instance methods are particularly bad – by what rationale does update_attribute not validate while update_attributes does??