Seen on FF Nightly 13.0a1 (2012-02-12): > var er = new Error('msg'); > Object.getOwnPropertyNames(er).sort().join('\n'); > Object.keys(er).sort().join('\n'); > var names = []; > for (var n in er) { names.push(n); } 2 > names fileName,lineNumber > ['fileName', 'lineNumber', 'message', 'stack'].filter(function(n) { return !!Object.getOwnPropertyDescriptor(er, n); }); fileName,lineNumber,message,stack According to getOwnPropertyNames() and keys(), er has no own properties. According to getOwnPropertyDescriptor() it has four. According to for/in, it has two enumerable properties, two of which are among the own properties reported by getOwnPropertyDescriptor(). This behavior is terribly inconsistent and confusing, leading to bad bugs in meta-programming code.

Pretty sure this is a dup of a bug on Error objects having no enumerate hook.