Narrative:

On the aforementioned date I was on a routine flight. We were in the climb about 500 ft below the assigned altitude of 6000 ft when a passenger in the copilot seat started to mess with GPS. I later found out that he was trying to help by entering the flight plan for the instrument approach to the destination into the GPS if we were going to need it. When he was entering the information he accidentally activated the flight plan. The autoplt at the time was following the GPS so the airplane started to do some turning to my surprise. I figured out the problem quickly and started to correct. When I looked up I was 400 ft high. The controller asked how high I was climbing and I responded that I was going to 6000 ft. He responded that he now had me level at 6000 ft, but that his radar showed me as high as 6600 ft. Everything was normal after that. Contribution to the problem was a non pilot trying to help, but doing more harm than good.

Google
 

Original NASA ASRS Text

Title: A BE58 PLT, DISTR BY A PAX, OVERSHOT HIS ASSIGNED ALT.

Narrative: ON THE AFOREMENTIONED DATE I WAS ON A ROUTINE FLT. WE WERE IN THE CLB ABOUT 500 FT BELOW THE ASSIGNED ALT OF 6000 FT WHEN A PAX IN THE COPLT SEAT STARTED TO MESS WITH GPS. I LATER FOUND OUT THAT HE WAS TRYING TO HELP BY ENTERING THE FLT PLAN FOR THE INST APCH TO THE DEST INTO THE GPS IF WE WERE GOING TO NEED IT. WHEN HE WAS ENTERING THE INFO HE ACCIDENTALLY ACTIVATED THE FLT PLAN. THE AUTOPLT AT THE TIME WAS FOLLOWING THE GPS SO THE AIRPLANE STARTED TO DO SOME TURNING TO MY SURPRISE. I FIGURED OUT THE PROB QUICKLY AND STARTED TO CORRECT. WHEN I LOOKED UP I WAS 400 FT HIGH. THE CTLR ASKED HOW HIGH I WAS CLBING AND I RESPONDED THAT I WAS GOING TO 6000 FT. HE RESPONDED THAT HE NOW HAD ME LEVEL AT 6000 FT, BUT THAT HIS RADAR SHOWED ME AS HIGH AS 6600 FT. EVERYTHING WAS NORMAL AFTER THAT. CONTRIBUTION TO THE PROB WAS A NON PLT TRYING TO HELP, BUT DOING MORE HARM THAN GOOD.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.