What are the assumptions made when code is written? Kuznetsov gives the example of a string search - what should happen if the string contains wildcard characters like square brackets or percent signs? You could ban wildcards, or you could write code to handle them. But doing nothing will give you a wrong result further down the line.
If you find a bug, look for it throughout your code - it has probably slipped through before. Fix problems pro-actively - it’s faster and easier than waiting for a problem to occur, then troubleshooting and implementing the same bugfix.
He describes how upgrades can screw up older code - code that works perfectly when using the read_committed isolation level may fail to work if someone turns on the new read-committed-snapshot option. There’s not much you can do about things that Microsoft haven’t released yet, but when they do happen you have to check their implications and change existing code.
I’m reading this as an eBook and it is hard to follow the sample code, but it repays a bit of effort. The key lesson is that it’s better to spend your time planning your code upfront than fixing cockups later.