Who'd have thunk it... if you plug your +/- 12v supply in backwards, the magic smoke gets out of the op amp!
I am lucky to not have lost other components (most notably the LDO regulator or the already-damaged AVR that I wanted to try using rather than throw it away). I have now added some diodes on the +/- input lines to protect against reverse polarity supply.
(Edit: turns out that it did destroy the LDO as well, it just took a bit longer until it completely gave up the ghost. The AVR is still hanging in there, though...)
I have some different colored cables on order, so hopefully this won't happen again. :-)