Sometimes the map that's leftover, that no else wants to decipher, is the most valuable to georeference.
In my periodic checks on BL Georeferencer http://www.bl.uk/maps/, I've noticed that the more opaque, difficult to discern maps are avoided - and this is entirely understandable! Associating the places that appear on historic maps with their current geographic location on the ground can be straightforward, but not always. Here are a few of those more demanding:
This 16th c. drawing required reading the title and description to get any idea of where or what it was - there is no text on the map!
This ca 1600 map of Shopshire is nicely labelled, but requires some interpretation. Shewsbury is labelled "Salop", an historical name for the city, and scale varies over the map: the loop of the river on which the city is located is of an inordinately large scale, but this emphasises its prime geographic feature making it firmly recognisable.
Thanks to Steven Feldman for taking on these two.
Below are a couple I am less certain will make it...
Numerous maps from the Kensington Turnpike Trust are available, and all seem to be difficult to move. This, sheet 6, has even less information than most!
No one has dared attempt this estate map of "the manors of Mincingbury, Abbotsbury and Hoares, in Barley, Hertfordshire." If anyone can figure where this might be located, please help!
What makes the difficult maps especially valuable to georeference is their very obscurity; because most folks will not know what they represent, they are made less useful. Once their location is known, they are able to be found and used as maps.
My thanks and admiration go out those participants in the BL Georeferncer project that accept the challenging maps!