c# vs2013 corrupt .dll

I have created a library for use in some of my applications. It loads information into various classes in memory and keeps itself updated.

Recently I updated it due to a small issue. This went through developer and UAT testing without an issue.

There is an issue where it goes from working correctly to throwing an error that one of the fields on one of the classes does not exist. Needless to say this does exist and is initialized correctly.

Part of the investigation was to start an application that uses the .dll into Visual studio. I put a breakpoint at a point where a function is called that uses the field that I have the problems with but when the problem exists I cannot step into the function for some reason. If I check the object before the function is called I can also see the field that is supposedly missing.

This function is called often without an issue until this particular issue decides to show itself.

How could a compiled .dll seemingly change behaviour after it has been working correctly for hours and what would stop me stepping into the code that uses it (a few lines into the function)?