In certain cases Zak keeps on walking while not moving it all. While playing the game I managed to trigger this bug on several places (see save-games 2,3,4) but I found a way to reliable reproduce it in the shop. Load the save-game nr. 9, enter 4 digits for the card. Zak will say walk to the front and say "Gee I hope I'll win". As soon as this text appears, wait 1 second and click on the shopkeeper. This will cause zak to walk to him. But before he is there the shopkeeper will say something and this cause zak to stop moving, but keep on walking.
I have tested this with ScummVM 1.0.0rc143718 on Windows XP SP3 and Zak-V1-C64 (English)
example 1
example 2
example 3
save-game to reproduce the issue
I can add a fraps movie of reproducing this if necessary
This may not be C64 specific, I tried Zak-FM and was able to trigger the same bug.
I also tested the V2 Amiga version and this has the same bug. The real (not VM) PC version does not suffer from it. I changed the bug to be more generic.