It's difficult to make it right (of one subtle error)

Our PVS-Studio analyzer has existed for several years. But we are still coming across "initial" errors in it, i.e. errors that have existed there for the whole life-cycle. We have checked the source code of the Chromium project recently (the archive with source code only occupies a bit less than 1Gbyte). This is a solution with about 450 projects. One of the projects would not get checked and generated strange errors at the preprocessing stage. We are using Visual C++ as the preprocessor, so we cannot analyze code without preprocessing.
Well, we found out that cl.exe incorrectly processes a line enclosed into double quote marks with a space. For instance:
/D "" WEBCORE_NAVIGATOR_VENDOR=\"Google Inc.\" ""
When cl.exe came across this command line argument, it processed it incorrectly as well as all the further arguments. And most likely, it is not the fault of cl.exe but such is just the approach to processing command line arguments in general. But it is not that important. What is of interest to us is that in order to recall this error, we must have a project that:

• has user define;
• and not just define but with a specified value;
• and not just a value but a string value;
• and not just a string value but with a space inside it;
• and of course it must be enclosed in quote marks.

Sure, this thing actually could (and must) have been provided for at the stage of developing, but it had not been done. And the error was detected only now.
Detecting such an error in a project with a long life-cycle means that it is very difficult to provide for every possible kind of external input data. This is a very hard yet interesting task.