After entering the Town, if you ring the doorbell twice and then enter the alchemist's lab, the coin is there. Then if you go to the grocery store, give the coin to the grocer and return to the alchemist's lab, the coin is still there (although it isn't in your inventory).
If you (after entering the Town) first give the coin to the grocer and then enter the alchemist's lab, the coin is not there.
Game version: DOS/English CD version 1.02 and DOS/English (GB) Floppy version 1.00 - both distributed by GOG.com
ScummVM version: Debian 64bit latest development version - ScummVM 1.7.0git37-g94fd380 (May 13 2013 05:54:32) - also tested on version 1.4.1
Ticket imported from: #3613221. Ticket imported from: bugs/6328.