This PDF, by Bret Boyd, takes the concept of scroll mishaps and applies it to magic items in general. Scroll mishaps occur when a user tries to cast a spell beyond his current ability (i.e. when his reach exceeds his grasp, although the opportunity is there for success). Substandard magic items come at this from the other direction - competent spellcasters use shortcuts and inferior components to arrive at a cheaper, yet fallible, magic item. These casters have the ability to create a normal item yet they either cannot or choose not to.
This PDF includes rules for creating, using, and identifying substandard magic items. Whether you're in a rush the next time you craft a magic sword or the DM sneaks an inferior suit of magic armor into the next pile of treasure the rules in this PDF can lead to fun, sometimes disastrous results.