Writing Efficient Ruby Code
Writing code is not typically atomic in nature. Even though a particular method may be, you have to think about the "big picture" in relation to the Module or Class and how it fits within your architecture. It's easy to lose focus on some of the finer details that could have a negative impact on the performance of your system.
These type of errors are usually unintentional and it's not that they aren't considered harmful, they just aren't considered. To discover some of the things you can do to write better code I recommend picking up a copy of "Writing Efficient Ruby Code" by Dr. Stefan Kaes.
The information value in relation to the cost of the book ($12.99) is very good and he does provide excerpts to entice you to buy. All in all, a nice job done.