A few people have reported their i3 suddenly "accelerating". Turned out in cases reported, that If the regenerative braking system was operating and the car made a sharp turn, or hit a pothole or a bump, causing the wheels to lose traction, the traction control would disengage the regen to prevent a loss of control. When this happened, it would give the driver the sensation of sudden acceleration. When you are in full regen and it suddenly disengages, it does feel like the car is suddenly accelerating or surging ahead, when if fact it just isn’t being slowed down by the regenerative braking as expected. The system was intentionally designed to do this, to prevent the thin tires from losing traction while negotiating hard turns or rough roads. It’s not a problem as long as you are aware of how the system functions, know it could happen, and you are ready to use the friction brakes if necessary.
I've encountered this myself, at a stop-sign near my house. Couple of roughly patched potholes leading up to the sign, hit at the right speed, cause the regen to disengage, and the car feels like it's jumping forward. I have always been a "two-foot" driver (rally car racing, left-foot braking is faster and safer on the course), so braking quickly never an issue, just comes automatically. I can see where it would be unsettling and might scare a one-foot driver, unless they know about it and are ready for it.
2015 i3 Rex, Capparis White, Tera World, Technology & Driving Assistant, Parking Assistant, Harman Kardon Audio System, 20 inch wheels, EVoInnovate EVSE