There's actually two ways you could interpret this that both make sense.
The only time it's NOT spelled incorrectly (double negative--meaning the only time it's spelled correctly) is when it's spelled I-N-C-O-R-R-E-C-T-L-Y. In other words, that's the only correct way to spell it.
OR...
The only time it's spelled in a way other than I-N-C-O-R-R-E-C-T-L-Y is if the spelling is incorrect. In other words, any spelling besides that is incorrect.
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