can not shift to park

Went to work like normal, didn't notice anything. Backed into my normal parking spot, shifting the stick to park, proceeded to grab my uniform from the back when I noticed movement out of the corner of my eye. Looking forward, I found my car rolling forward towards another parked car. I immediately braked, turned the car back on and reversed back. Put the car in park and everything was fine. Problem occurred once again the very next day. This time noticed the gear shift box had all the lights lit up when normally only the gear that's engaged is lit up. When placing the car in park, the gear would not shift and the car would roll unless the emergency brake was applied. The problem later worsened to not being able to shift from park without having to shake and smack the stick several times. Had a family mechanic look at it, he immediately found the problem upon popping the hood. A tiny washer had popped out of place on the assembly arm for the gear shift. He pressed it back into place and secured it with electrical tape to ensure it would not pop out again and the problem was solved.