I remember a Transair A300 taking off in MAN with the gear pins in (1997), they dumped 15t of fuel and returned to MAN.
Back to the original post, I spoke to a TK crew and they told me that the crew was on their 3rd leg that day. The gear has been left down to cool the brakes. Wx was bad and they had some sort of serious malfunction which required their immediate attention.
This, of course is all second hand info only (rumour) and I knew comments like the one from 'Forget' would rain down on me. But hey, they really seemed to have forgotten the gear while they where extremly busy sorting out the other problem(s).