Undefined behavior is a characteristic of certain programming languages (most prominent in C and C++) to produce a result in certain situations that depends on compiler implementation or specified optimization switches. In other words, the specification does not define the language's behavior in all possible situations but states: "in case of the A condition, the result of B operation is undefined". It is considered a mistake to allow such a situation in your program even if it is executed correctly with some particular compiler. Such a program won't be a crossplatform one and may cause failures on a different computer, operating system and even with different compiler's settings.
You should not mix up undefined behavior with unspecified behavior, the latter being the case when the specification does allow not every behavior possible but a restricted range of implementation variants.
Below are examples of situations causing undefined behavior:
- Using a variable before initializing it. Undefined behavior occurs when trying to use the variable.
- Memory allocation using the new  operator and subsequent release using the delete operator. For example: T *p = new T; delete p;. The correct code is: T *p = new T; delete  p;.
- A variable is changed several times within one sequence point. As a canonical example, the i=i++ expression is often cited where assignment of the i variable and its increment are performed at the same time. To learn more about this kind of errors, read the section "sequence points".
- Wikipedia. Undefined behavior.
- Wikipedia. Sequence point.
- Terminology. Sequence points. http://www.viva64.com/en/t/0065/
- Danny Kalev. Undefined Behavior vs. Unspecified Behavior. http://www.devx.com/tips/Tip/12684
- John Regehr. A Guide to Undefined Behavior in C and C++. http://blog.regehr.org/archives/213
- The Old New Thing. Undefined behavior can result in time travel (among other things, but time travel is the funkiest). http://blogs.msdn.com/b/oldnewthing/archive/2014/06/27/10537746.aspx
- John Regehr. C Compilers Disprove Fermat's Last Theorem. http://blog.regehr.org/archives/140
- Andrey Karpov. Null Pointer Dereferencing Causes Undefined Behavior. http://www.viva64.com/en/b/0306/