I have a rather large application that acts like a server.
However, we've just found a bug in it and we'd like to get it fixed.
What we need is to create a hook to a specific set of functions in order to do some additional checks.
If the data sent to those functions pass those checks, follow the normal course of the function.
Else jump to the area of the function that handles errors.
We won't be providing the application, but we'll provide any other additional data you need for the project to succeed.