I'm seeing a problem with the function LetterNumber. When I evaluate it (for example LetterNumber["c"]), sometimes the function never returns and I have to break out of the evaluation with Alt-,. One one system it works fine all the time, on another two that I tried it on it worked sporadically or not at all. This is happening with Mathematica version 10.3.0.0 under Windows 10, 64 bit. Is anyone else seeing this behaviour? I thought it might have been some kind of corruption in a particular notebook, but the same thing happens when I try and evaluate it in the help notebook for the function.