Clever tool to bail out if you do press the pin too far out. Some chain tools (Park's CT-3 for example) have a travel limit stop to keep you from pressing a pin all the way out of the chain's sideplate for chains with reusable pins.

Also, all modern 7/8-speed and above derailleur chains must not have an original pin reinstalled whether it's pressed partially or all the way out. The old pin must be removed and replaced with a specific joining pin or a master link.