AIF exception error “Instances of abstract classes cannot be created”

The AIF exception errors normally are quite self explanatory; especially validation but recently we were faced with the following odd error: -

The request failed with the following error:  Instances of abstract classes cannot be created

What is very odd about this error is that we knew we didn’t have any code that was trying to instantiate an abstract class. After all we had no compile errors.
So what could be causing such a strange error? My immediate thought was that it was so odd that it could be a corruption of some kind. The most likely corruption would be an IL build one and this was indeed the case. We executed a full IL build and once that had been done we were able to successfully import data via AIF again without any error.
So if you are ever faced with an AIF exception error “Instances of abstract classes cannot be created” I would recommend you first check your IL build.