I had a case of bad math on my chem test... lost 18 points because of one question that I fucked from the beginning, I read the number of H bonds wrong, so i messed up the lewis structure. that in turn made me get the totally wrong arrangement of the lone pairs, the right bonds, and ended up with an octahedral shape instead of the trigonal bipyramidal i should've gotten. Structure being big enough for LDs, calcualted wrong relative melting/boiling points, and got the totally wrong conclusion.
But i got the initial multiple choice part right, which let me have a 40%.