hypomagnesemia
"I took magnesium glycinate for months with no problem, when I ran out of it I bought this one (magnesium oxide) because it was alot cheaper. Pretty soon I started having a weird cramp in my calves when I walked.  I stopped taking it for two weeks, no cramps. Started it again and had cramps immediately (only when walking). 

So I went ahead and bought the more expensive glycinate, and sure enough I haven't had any cramps.  Weird!  But I won't take this one ever again."