It’s easy to delude yourself when writing software. Do these tests really describe what the application does? Does the documentation really describe how the system works now? Is this comment an accurate assertion on the state of affairs in the application?
My experience is that there’s little to solve this problem besides discipline. Always double check that you haven’t invalidated something that was written down in the margins. If there’s a way to encode something in code instead of prose, do it.
Vigilance against future-lies is an ever-mindful challenge.