In Rails 4 and below, callbacks like before_validation always had a gotcha that you had to watch out for:

before_validation :ensure_field_is_false_when_condition

def ensure_field_is_false_when_condition
  self.field = false if condition?
end

When condition? is true, the assignment happens. However, assignment returns the value that was assigned, so the before_validation receives the returned value false. As the docs mention:

If the returning value of a before_validation callback can be evaluated to false, the process will be aborted and Base#save will return false.

This is usually handled by an explicit return (i.e. nil does not stop the callback chain):

def ensure_field_is_false_when_condition
  self.field = false if condition?
  
  # extraneous return that is not Ruby-ish
  return
end

Rails 5 Update

There is a great change to the before_validation callback behaviour in Rails 5:

If the before_validation callback throws :abort, the process will be aborted and ActiveRecord::Base#save will return false.

This makes halting the callback chain much more transparent and intention revealing:

before_validation :safely_assign_false, :always_halt_validation

def safely_assign_false
  self.field = false
end

def always_halt_validation
  throw(:abort)
end