For many developers, concurrency remains a mystery.
Concurrency is the system's ability to act with several requests simultaneously, such a way that threads don't corrupt the state of objects when they gain access at the same time.
A number of good books have been written on this subject, such as Concurrent Programming in Java and Java Concurrency in Practice. They deserve much attention, since concurrency is an aspect that's hard to understand, and not immediately visible to developers. Problems in the area of concurrency are hard to reproduce. However, it's important to keep concurrency in mind to assure that the application is robust regardless of the number of users it will serve.
If we don't take into account concurrency and document when and how the problems of concurrency are considered, we will build an application taking some risks by supposing that the CPU will never simultaneously schedule processes on parts of our application that are not thread-safe.
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