Potential navigation with GPS include (but are not limited to):

• As AKSAR says, database issues are undoubtedly the top problem that people face. This is a complex topic and I am not an expert on it.
• Reception issues, including low signal, reflected signals, and interference.
• Device software and hardware issues.
• GPS signal spoofing.

I use GPS constantly and often as my primary navational tool, but the human brain must make the decisions without trusting a single tool so much as to go places one should not.