When an expression contains too many nested calls to logically process within a flow, the system returns an error message. An expression exceeds limitations when it goes over 60 calls deep in one place. 

Note: Nesting calls too deeply in a single expression increases the chance of runtime error. For example, the following expression is 3 calls deep: 
Append("abc", Append("abc", Append("abc", "123")))

If your expression includes 60 or more calls, the system will not process it and returns an error indicating that it is nested too deeply.

Consider evaluating your expression or flow design if you receive this message. Best practice recommends that you create smaller expressions to hold temporary results of the intermediate expressions, and then use the smaller expressions in a final expression.