I've found a lot of Rails apps over the last couple years that were diligent about having an attr_accessible in every model, but not diligent about what went in the attr_accessible. Following the Rails idiom, they were doing all their attribute assignment through update-style params[:model] model[foo] model[bar] stuff, and attr_accessible "breaks" that.
The Rails tutorial is good (and ambitious) --- just know, this stuff trips up solid, experienced Rails developers all the time.