I've found that quite a lot of cheap keyboards cannot register a T when shift-R are being pressed. If I always used the standard QWERTY finger positioning, this wouldn't be a problem, because my index finger could not be on both R and T at the same time. But when typing a word like (all-caps) SMART, I tend to use middle-index to quickly type the RT, and R is still depressed when I hit the T. So the T does not register.

Most decent keyboards don't do this, but even there I've seen exceptions. Very annoying.