Dwarf Fortress Bug Tracker - Dwarf Fortress
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0002820Dwarf FortressPathfindingpublic2010-07-24 23:262018-01-19 16:22
Toady One 
normalminorhave not tried
0002820: HFS creatures can emerge through vertical diagonals without HFS being officially breached
Spoilers ahead

Aware that I was approaching Hell while mining adamantine, I dug a stairway down a fair distance and stopped, mined out all the nearby adamantine, and decided to mine another vein I could see through the magma sea. After accidently breaking into the HFS, I was surprised and Doomed when the Demons came out the other stairway which had not opened Hell.

The problem is that digging a stairway like this (Sideways):

(Where X is a stairway, and there is a floor between the open area at the top and that which leads to suffering) does not trigger the HFS, but can be used by the floods of horror to escape anyway.
Find a point where you might find Hell

Find the exact spot which opens it, and reload

Dig an up/down stairway just to the upper edge of Hell

Open Hell elsewhere, and enjoy
related to 0008383resolved Toady One Flying/swimming/climbing creatures can move vertically through diagonal gaps containing down-stairways 
Issue History
2010-07-24 23:26WarlemmingNew Issue
2010-07-24 23:29WarlemmingNote Added: 0010944
2010-07-24 23:29WarlemmingTag Attached: HFS
2014-07-27 12:48FootkerchiefRelationship addedrelated to 0007130
2014-10-01 15:58FootkerchiefRelationship addedhas duplicate 0008383
2014-10-01 18:14FootkerchiefSummaryHFS does not open on slight x-axis difference, but HFS things clipping through corner => HFS creatures can emerge through vertical diagonals without HFS being officially breached
2014-10-01 18:14FootkerchiefRelationship replacedrelated to 0008383
2014-10-01 18:14FootkerchiefAssigned To => Footkerchief
2014-10-01 18:14FootkerchiefStatusnew => confirmed
2014-10-01 18:15FootkerchiefRelationship deletedrelated to 0007130
2015-01-08 18:23lethosorDescription Updatedbug_revision_view_page.php?rev_id=12728#r12728
2015-01-08 18:24lethosorNote Added: 0031798
2015-08-20 10:00PatrikLundellNote Added: 0032976
2018-01-19 16:22Toady OneStatusconfirmed => resolved
2018-01-19 16:22Toady OneFixed in Version => Next Version
2018-01-19 16:22Toady OneResolutionopen => fixed
2018-01-19 16:22Toady OneAssigned ToFootkerchief => Toady One

2010-07-24 23:29   
The picture didn't come out right because of the dots, the X is meant to be just above to the right of them.
2015-01-08 18:24   
Wrapped the picture with < pre> and < /pre> (without spaces after the opening brackets).
2015-08-20 10:00   
The issue is not restricted to HFS breaches. There is no report for either the caverns nor the magma sea when they are breached diagonally downwards by a staircase (especially nasty when you dig straight down and hit SMR, which just causes the miner to stop with no report, and you later discover magma creatures inside your fortress).