Mathematica is misinterpreting input from the keyboard, as if I had chosen some other keyboard layout.
- [ is interpreted as @,
- ] is interpreted as uppercase D,
- £ is interpreted as o with a diacritical mark of some sort, and
- ( is interpreted as uppercase H.
In fact the behavior is even weirder: if I type two open parentheses, then the response is to first display an uppercase and then to insert an uppercase K in front of it (not even at the location of the blinking cursor). Also the behavior sometimes disappears, so that [ indeed produces