The supercombiner is the letter "u" combined with the first hundred combining diacritic characters in Unicode.
This should render as a "u" with lots of diacritics above and below it in a large stack, but since this is a pretty difficult rendering challenge, many programs of course get it wrong.
Mozilla Bug 403464 (alias, supercombiner) was the first major incorrect rendering of the supercombiner to be submitted, by Justin Dolske on 12th November 2007. The case had been mentioned to him by my friend Pat Hall, who saw it as a great Unicode smoke test.
Firefox and Safari failed spectacularly when the original Mozilla Bug was filed.
One of the best supercombiner errors came in writing this page. I accidentally deleted the last few combining characters of the supercombiner, and when I tried to use the Undo function in TextWrangler, it complained with "(MacOS Error code: -116)", i.e. Size Check failed, and simply wouldn't restore the diacritics.
In Google Chrome 5.0.375.99 the supercombiner renders correctly when a sans-serif font-face is used, but not when serif is used.
Created with the following python code:
s = 'u' for i in xrange(1, 3000): if unicodedata.category(unichr(i)) == 'Mn': s += unichr(i) if len(s) > 100: break
There are also Linux and OS X clipboard / pasteboard scripts.
Messages for Mac gives some weird errors: (a), (b), (c)
Sean B. Palmer · Updated 18 June 2012