It is easy to forget that a symbol has been defined to be some expression. If you use that symbol later, you can get surprising results. For example, if you define a = x2, forget about it, and later compute f (a) for some function f that you have just defined, you are in for a surprise. In complicated computations the error may not be apparent.
Tip Check the Show Definitions list from time to time. If your mathematics is behaving strangely, this list is a place to look for a possible explanation.