I've encountered a map bug in 1.6.4 SMP, which is similar to this one explained [in the wiki](http://minecraft.gamepedia.com/Map#Bugs):

> 
> Currently, placing a map on a wall in SMP using an Item Frame possibly
>  causes an issue with maps held by players in which the location marker
>  for the players 'disappear' under explored regions of the map, and
>  preventing unexplored regions from becoming 'explored', effectively
>  making all maps on the server useless. Deleting all of the map.dat
>  files in the world data folder, as well as the idcount.dat file
>  resolves the issue, unless players have maps in their inventory.
> 
> 
> 

The difference is that despite not showing my marker, it *does* explore new areas, contrary to what is written there.

Also, *"Deleting all of the map.dat files in the world data folder, as well as the idcount.dat file resolves the issue"* is kind of vague: it may solve the issue, but does it cause any problem? Or are these only cache-like files?

PS: [reported](https://mojang.atlassian.net/browse/MC-33254)