function f(x) {
    print("1st: ",
        evalcx("(function() { return " + x + "; })()",
            newGlobal({
                sameZoneAs: null
            })));
    print("2nd: ",
        evalcx(x,
            newGlobal({
                sameZoneAs: null
            })));
}
f("this.y += 'ONCE';");

prints the following different (NestTest mismatch) output when they should possibly be the same, i.e. ONCE should only be printed once:

1st: undefinedONCE
2nd: undefinedONCEONCE

This may be a regression from m-c rev 450b8f0cbb4e and hence bug 1518753. Jan mentioned to file this over email. Tested on a 64-bit debug shell on m-c rev 8ec327de0ba7

===

$ ./js-dbg-64-linux-8ec327de0ba7 --fuzzing-safe testcase.js
1st: undefinedONCE
2nd: undefinedONCEONCE

$ ./js-dbg-64-linux-8ec327de0ba7 --fuzzing-safe --more-compartments testcase.js
1st: undefinedONCE
2nd: undefinedONCE

https://hg.mozilla.org/mozilla-central/rev/6ec84030fb70