Miscalculation (5)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">miscalculation</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for error that I've seen before include "Blunder or mistake" , "Boo-boo" , "Bug" , "Miscarriage of justice" , "Howler" .)
(Another definition for underestimation that I've seen is " deficient appraisal".)