The two achievements that are unobtainable are Notorious World Saver and The More the Merrier.
Notorious World Saver- "The achievement „Notorious World Saver“ has been removed from the game due to a bug which allowed players to obtain this achievement without any effort - A post written on 14.12.2011 by a CM on the official Tibia forums"
Source: Tibia Forums: Notorious World Saver
The More the Merrier- "This achievement used to be of grade 1 and give 3 points, but the backlash CipSoft faced for implementing an achievement involving real-life expenditures urged them to change it into a worthless curio on September 15, 2015. However, it can inexplicably still be earned by anyone willing to satisfy the requirements (albeit to no effect). Only players who earned it prior to CipSoft's rectification can select to display it on their character page, though."