During gta3 the game was buggy during most of its development. This happened because we started the game from scratch and also used a new rendering engine (RenderWare) that was also not all that mature at the time.
Through most of development, it was rare to play through a mission without a crash happening. The frame rate was below 10 for most of the time and the map was full of holes.
Later we developed a bug database but during gta3, the testers would simply print out every bug. Every programmer, level designer and artist had a physical pile of sheets on their desk. If you felt the bug was more easily fixed by someone else you would simply put it on that persons pile.
The collision in a game is usually different from the visual polygons. This is because collision detection is relatively slow. A building with 10,000 visual polygons may have only 1,000 collision polygons. As a consequence, the map may have holes that you cannot see. This is what's going on when you fall through the map.
This makes it very hard for the testers to test the collision on the map. During gta4, the testers found it particularly hard as characters could fall through the map depending on their animations and there was no way they could touch every bit on the map.
This is why they asked for a rag doll gun. Alexander Roger created a special gun that instead of bullets would fire rag doll characters. The testers could go round the map and fire rag dolls at every building and hill side.
The testers are the unsung heroes here. Whenever they would find a bug they would try to replicate it several times. They would try again with different vehicles or different weapons. They would produce a detailed report to be sent to the artists, coders and level designers. It must have been very boring. I'm still not sure how they stayed sane and awake.
If I remember correctly, we had around 70,000 bugs for SA. For IV it was almost double
Whenever a change is made, there is a real chance a bug is introduced. Sometimes there wasn't much time between a change being made and a release to be sent to New York. The testers used to do speed runs to make sure at least no bugs were introduced that would stop the game from being finished. For gta3 they took 1 hour and 20 minutes. Pretty good considering the record today is only just under 1 hour.