The jack is one of the oldest and most familiar pieces of hydraulic equipment. This familiarity contributes to misuse which can lead to personal injury. The jack is a load lifting device and should never be used as a load holding device, especially when a person will be going underneath the supported load.
It is therefore very important that you use your jack correctly and with great care. There are many specialist jacks where you really ought to have proper training in order to use them but even if you are just using a hydraulic car jack to change a wheel, read the instructions carefully and take care when using it.