Take this scenario: after a compile from scratch of a Cocoa Touch project, Xcode may report several compiler warnings in different files.
If I fix the warnings in only one of the files and hit Build to see if it works, Xcode will no longer report any warnings in the other files (it will continue showing those in the currently open file if they’re not fixed). If I manually switch to any of the other files I remember having warnings in, I can still see them reported correctly between the code lines. However they are not summarized in the Build Results screen, so I can’t see an easy way of getting to these files.
This is normally a minor issue. But after a recent refactoring that produced ~50 warnings in about 20 files the problem started to get annoying.
When you made your changes, X-Code did not build your entire project again, only what it needed and hence displayed only the warning relvant to the latest build.
If you go to the build results window command-shift-b you can configure it show you either latest results or all results.