Buildings, cliffs, valleys are a problem, but sometimes less than expected.
Try using the building, cliff or valley as a navigation aid!
Trees covered in sap-laden leaves can block the satellite signals.
Deadly! And just when you really needed some help!
Deep layers of falling drizzle. The satellite signals are weakened and the GPS may lose them.
However, please do not blame your GPS for apparent location errors if you are using maps other than UK Ordnance Survey or French Institut Geographique ones. In the rest of the world you will find your GPS is a bloody sight more accurate than the maps are!