With a ramp in the bottom row on the second peg in from either side, facing the center, balls have a tendency to bounce over the middle and release the wrong coloured ball maybe 1 time in 3. Fix: turn the ramp around so the ball rolls uphill and back down, so it’s stopped bouncing before it gets near the center. Alternate fix: have a component on the bottom/center peg so the ball bounces off.
Gear bits are unreliable on the top row. Anywhere else, the balls are moving on a suitable path to cope with the gear bit not quite being fully to one side or the other, but the source ramp has a tendency to find the crack when the gear bit is meant to be pointing out, but is a bit off. Maybe a 1 in 5 chance. Fix: make sure the balls go through another component before hitting a gear bit.
Things also occasionally go haywire in non-reproducible ways but those two are the ones that are reliably unreliable.