I have run into numerous occurrences where the focus (i.e. cursor position) in the Script workspace will spontaneously jump around. It isn't a case where a script step itself is moving (I have seen reports of that, but this isn't that).
The most frequent case is that I will be typing a comment text...type type type type...boom! The comment line is exited, and a new line is started, typically at the very end of the script, and my next few characters show up in this new line. This happens at both my laptop and a desk with an external keyboard, so it isn't just a case of accidentally touching the laptop touch pad.
This is OS X 10.9.5; FMPA 14.02.