###!!! ASSERTION: Adding a child where we already have a child? This may misbehave: 'Error', file docshell/shistory/src/nsSHEntry.cpp, line 703

Hit this assertion leading tinderbox on a recent pull: --DOMWINDOW == 136 (0x7fd5b068) [serial = 2122] [outer = (nil)] [url = https://bugzilla.mozilla.org/show_bug.cgi?id=566066] WARNING: 1 sort operation has occurred for the SQL statement '0x91db9c08'. See https://developer.mozilla.org/En/Storage/Warnings details.: file /home/jesup/src/mozilla/head/storage/src/mozStoragePrivateHelpers.cpp, line 144 WARNING: Subdocument container has no frame: file /home/jesup/src/mozilla/head/layout/base/nsDocumentViewer.cpp, line 2421 ++DOMWINDOW == 137 (0x7fd5bd68) [serial = 2157] [outer = 0x83d55020] --DOMWINDOW == 136 (0x83d5ad08) [serial = 2156] [outer = 0x83d55020] [url = http://tinderbox.mozilla.org/showbuilds.cgi?tree=Firefox&panel=1] ++DOMWINDOW == 137 (0x7fd5c248) [serial = 2158] [outer = 0xaa0eb160] pldhash: for the table at address 0x83b09c68, the given entrySize of 48 probably favors chaining over double hashing. ++DOCSHELL 0x83b09c00 == 61 ++DOMWINDOW == 138 (0x7fd5ca68) [serial = 2159] [outer = (nil)] WARNING: NS_ENSURE_TRUE(aURI) failed: file /home/jesup/src/mozilla/head/content/base/src/nsContentUtils.cpp, line 4985 ###!!! ASSERTION: Adding a child where we already have a child? This may misbehave: 'Error', file /home/jesup/src/mozilla/head/docshell/shistory/src/nsSHEntry.cpp, line 689 WARNING: Subdocument container has no frame: file /home/jesup/src/mozilla/head/layout/base/nsDocumentViewer.cpp, line 2421 ++DOMWINDOW == 139 (0x7fd5b208) [serial = 2160] [outer = 0x7fd5ca20] --DOMWINDOW == 138 (0x7fd5bd68) [serial = 2157] [outer = 0x83d55020] [url = http://tinderbox.mozilla.org/showbuilds.cgi?tree=Firefox&panel=1] --DOMWINDOW == 137 (0x83d55068) [serial = 2155] [outer = (nil)] [url = http://tinderbox.mozilla.org/showbuilds.cgi?tree=Firefox&panel=1] --DOCSHELL 0x83b71600 == 60

Should we try fixing this assertion or make it a warning again?

I see this a lot in the tests I am working on for bug 341604 (iframe sandbox) fwiw.

I'll look at this.