We all know (or should know) the medical abbreviations you're never supposed to use:
-- QD, QOD
-- trailing zero
-- MS, MSO4, or MgSO4
However, there are a few additional abbreviations that we're supposed to avoid because they might be confusing:
-- greater than or less than symbols
I actually was informed by a nurse once that I had to rewrite an order because I wasn't allowed to write "straight cath for volume > 300 ml". Apparently, this could have been mistaken for "straight cath for volume 7300 ml. Because that's a totally reasonable mistake that someone could make. Please straight cath patient if their bladder has 10 times more urine than a normal bladder can hold.
Of course, if your handwriting is crap, it doesn't really matter what abbreviations you use. When I was rotating through the ED, I got yelled at by an attending for writing an order to give a diabetic patient D5 NS. What did I actually write? 0.5L NS.