I do similar checking with 10 compilers, and consequently, I have spotted more than thirty compiler bugs in the past four years. They were submitted to, confirmed by, and sometimes fixed by vendors. In addition, I maintain a catalog of them at GitHub:
In this process, I find that NAG is the second most responsive compiler vendor to bug reports. The first is/was Absoft, which unfortunately ceased operation a few years ago.
At the very beginning, I was astonished to find bugs in compilers. Then it became my daily life. Many people are surprised when I claim that I have spotted quite a few bugs in reputable Fortran compilers (and even in MATLAB). I tell them that they will find the same if they test code as intensively as me.