(Also present in r6943.)
Best understood if I just show you how to recreate it:
1) Create a powerline connected to a power grid that is being used.
2) Drag a bridge that ends on the powerline (the pop-up shows that an invisible powerline still exists on the bridge's end tile):
(http://i.imgur.com/7KdBSuF.png)
3) Delete the invisible powerline - CTD occurs, terminal shows a segfault occurred.
Do this with an inactive power grid and nothing abnormal happens.
Cheers guys, hope this helps. :D
EDIT: Works with power bridges too; trying to delete the end tile here causes a segfault.
(http://i.imgur.com/eaKpEaX.png)
Try next nightly please.
Cool, fixed in r6999. Bridges can't end on a powerline tile now, as would be expected. Thanks guys! :D
(BTW, it may have already been fixed earlier before I posted, but the nightly build after r6993 was a messed up download. Sorry!)