Mapping validation fails in the following situations:

The transformation contains stateful variable ports.
The transformation contains unsupported functions in an expression.

If you use a port in an expression in the Aggregator transformation but you do not use the port within an aggregate function, the run-time engine might use any row in the port to process the expression.

The row that the run-time engine uses might not be the last row in the port. Processing is distributed, and thus the run-time engine might not be able to determine the last row in the port.

You cannot optimize the data cache for the transformation to store data using variable length.