The Jargon File (version 4.4.7, 29 Dec 2003):

misbug
 /mis?buhg/, n.

    [MIT; rare (like its referent)] An unintended property of a program that
    turns out to be useful; something that should have been a bug but turns
    out to be a feature. Compare green lightning. See miswart.


The Free On-line Dictionary of Computing (30 December 2018):

misbug

   /mis-buhg/ [MIT] An unintended property of a program that
   turns out to be useful; something that should have been a
   bug but turns out to be a feature.  Usage: rare.  Compare
   green lightning.  See miswart.

   [Jargon File]