debugger <Restart> vs <Through> "Code simulation error"

I'm not sure if this is bug worthy or just a curiosity...

In Workspace execute...
    self halt.
    x := 1.

Then click <Debug>, <Through>, <Through>, <Restart>, <Through>
to observe a "Code simulation error"

cheers, Ben
( Pharo-1.4 #14333 / cogwin_r2522 / Window 7 )