My dupes built 5 tiles high - screenshot - explorer crash

Ok my dupes built 5 tiles high, i was wondering about, wanted to start a thread about it "since when dupes could build 5 tiles high?", but when i took a screenshot of it, explorer crashed / system restart (ONI running fine/not crashed "only" explorer"), no screenshot in clipboard.
The posted (build overlay) screenshot is from the second try (reproduce), but everything was built normal (5th row not touched (unreachable)).
I run a Win 7 OS

Edit: Here is an area where they finished building 5 tiles high