Not planning 
Not checking computer-based flight planning tools against a chart, performance numbers, or weather briefings (Sure, both departure and destination airports are VFR, but there is a cold front with squall line thunderstorms between the departure and destination airports) 
Using any one tool at the exclusion of others 
Thinking GPS solves all navigation problems 
A good pilot uses more than one method of navigation, at least three when in high density airspace.