Before I mention some of the examples, let me give you an introduction to deterministic and non-deterministic formula fields. Formula fields whose value will be static are referred to as deterministic fields. Whereas, formula fields whose value will be changed dynamically or whose values will have to be calculated on the fly, they are referred to as non-deterministic formula fields. A classic example of that is a formula returning the current date and time.
Some examples of non-deterministic fields in Force.com are:
Formula fields whose reference spans over other entities
Fields having dynamic date functions like:- TODAY() or NOW()
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