Whatever the error, it's still present in 40c. All I can work out is that:
a) It can fail on the first ever attempted use, but that must be a fluke (I can get a good half hour or more gaming under my belt before it fails),
b) It doesn't actually have anything to do with suspension or items blocking the construction site. Note that in the stages of the game where this all happens to me I rarely have to use the Job List view to toggle things and correct matters such as suspension of jobs, so I
think every instance has been when using the zoom function from the Unit List (which I find far more useful to me during that period of time). Thus it specifically excludes suspended building tasks in the first place (although it's also much easier to ensure that nothing /does/ get suspended, at least while wild animals and enemies are not impacting upon normal operations).
c) It applied equally to building walls or floors or possibly even up/down staircases, with wood or stone (though never imagined that to be a relevent distinction), but I've never noticed it happen for any Construct Building that is (frexample) a farm plot or workshop. This could be just because (farm plots aside) I generally only build one each of a few of the workshops near the beginning, whereas prospective walls and floors can be quite numerous. I
have had it crash on attempting to zoom onto a Store Item In Stockpile target, on one occasion. I have
never had a problem with zooming to a creature, and I use that a lot (especially if I've been trying to avoid building zooming).
c.2) I've never had it crash on using the "q" option from the 'normal' passage-of-time mode, which brings you to the same mode ("Set building tasks/preferences"?) that the Zoom-Bld should, albeit cursor-aimed rather than zooming.
d) Saving of a world might have an effect (when it was failing on first use, consistently, saving the game before using Zoom-Bld the first time delayed the next crash upon re-opening way beyond the first ever use of Zoom-Bld) but I think that's just chance, not any real causality.
e) No world I created since 30f has escaped this problem (Pocket to maximum sized and parameter customised), and it has happened with an imported save from a pre-30f world.
f) It doesn't seem to depend on the rapidity of keystrokes. I've had it freeze when cautiously pressing the "b" as well as the usual "(U)nit List/<cursor*N>/Zoom-(B)ld" combo as quickly as I will usually tap out.
g) The XP error reporting dialogue doesn't have any
obvious useful points of common reference.
(And to follow up with the off-forum message, Toady, I did get a write-up (from world creation through to crash) of a 40a Pocket world generation that ended up exhibiting the problem. Too long to bother people with on here, but it's on offer. Or something from 40c if you're willing to wait, or whatever version we're up to by the time I come back from a long weekend away.
)