I was using Tweak on a single isolated sheet (connected to rest of protein by loops, sheet was 1 long and not hydrogen-bonded to anything).  The axial arrows (rotate about the protein's axis) worked as expected, but the arrows pointing in the other direction (pointing towards the loops attached to it) left me with a blank Internet Explorer window and this problem.  It didn't save or anything, it just abruptly transformed.  Windows Task Manager confirmed that the process became iexplore.exe instead of foldit.exe.