No cold-press juicer likes being rushed. Jamming in more fruits or vegetables while it is already full won't make it juice any faster and can in fact cause problems. Most vertical slow juicers have a built-in safety mechanism to stop the motor before this can occur. However you must be more careful with horizontal juicers, as they don't have this capability. Generally a horizontal slow juicer won't jam, but the excess juice will start backing up into the feeding tube. In the worst case, it could seep through the gasket and into the motor.
The solution is simple: slow down. Most vertical slow juicers are self feeding, so just feed one piece of produce at a time and wait for it to process that before dropping in the next piece. With carrots, hold the carrot and let the vertical take little "bites" off of it instead of dropping in an entire fat carrot. With horizontal juicers, if you see the juice begin to back up, slow down. Let it process what it already has in the juicing chamber. After the juice is mostly extracted, slowly feed more produce. This is especially true for soft fruits like pineapple or strawberry.
Besides being better for your juicer, feeding produce slowly has another benefit: you can get up to 15% more juice if you give your jucier the time it needs to properly process it.