Human error is inevitable. What is important is to ensure that human error does not result in adverse events such as air accidents.
This can be addressed in two ways: reducing errors in the first place, and controlling errors such that they, or their immediate effects, are detected early enough to allow remedial action. CRM addresses both types of mitigating strategies, but concentrates particularly on error detection, especially in the multi-crew situation.
Human reliability is the science which looks at the vulnerability of human beings to make errors (or less than perfect performance) under different circumstances. One could argue that it is more of an art than a science, since it is very difficult to predict, in quantifiable terms, human reliability in different situations, and from individual to individual. However, there are certain conditions under which humans are more likely to make errors (e.g. during circadian lows, when stressed, when overloaded, etc.).