I would recommend to have a look at
ABAP-Programmierung für den Vertrieb mit SAP – Kundeneigene Erweiterungen - User-Exits und BAdIs, Hunger, Petra; Klein, Thomas
Rheinwerk, 2012, Gebunden, ISBN: 9783836219341
Sadly I only found it in German, but as starting point it is a huge help for people who are at the same point of job-related development as you are and want to get deeper with the described topic.
*... who can do field replacements in the debugger can do anything in the system
*so this check can not stop (him) anyway.
