The following is a list of stuff not to do when navigating with GPS
1) Do not place waypoints on solid objects. This can lead to collision with said objects.
2) Do not "Drive the TV". This is a situation where a skipper sits at the nav station and stares at the pretty boat on the screen and does not consider outside information. The worst case of this is the powerboat skipper who sits on his flybridge at night with all the interior lights on and is not able to even see outside. He just stares at the "TV" and does not keep any semblance of a proper lookout.
3) Do not assume the chart information displayed on your screen is correct. Most chart software suppliers only provide updates to their data every year or two, where paper charts can be updated regularly by hand. This can be an issue when depths change in an area of high silt deposit or other man made alterations to nav aids. Also most electronic chart data is digitized from another source. This is an opportunity for human error.
4) Do not assume GPS will always be available. Murphy's law is an important consideration at sea. Your GPS can fail from something as simple as lack of power, physical damage, or corrosion. The American government can also de-activate selective availability and reduce your GPS from 10m accuracy to several hundred meter accuracy when ever they feel like it. The safest option is to be prepared to take over from the chart plotter by recording regular fixes in your log or on a chart. To do this you also need to have paper charts, navigation tools including a hand bearing compass, and the skills to use them.
In short, having a GPS aboard does not mean that you no longer need to know how to navigate by hand and eye.