I actually did read Witches Abroad after Lords and Ladies, but thanks to the Internet and the Discworld Companion, I wasn't exactly lost.
In fact, the last Discworld book I ever read before having read every book in continuity was actually Equal Rites, although I forget whether I had read Wintersmith before or after it. But after that, I've read each one that's come out in sequence, though that's only three books (Making Money, Unseen Academicals, and I Shall Wear Midnight).
I think the first ever Discworld book that I ever read was Soul Music, as I had played a demo of the PS1 version of Discworld II (but never got around to buying the full game, and probably won't get a chance now) and had seen the animation of Soul Music.