Elektrik: Disconnection Mass lead

[Beumers]

Hi.
When working on the electric system, in the manuals there is very often a warning to disconnect your Mass lead. Every time you do so afterwards you have to set your clock again ect. I always do it just to be sure, but in the past I disconnected electical components with the current on and nothing went wrong. I just didn't know.
I took the LKM out of the car ( resolder ) and I disconnected the mass lead acording to the manual. What can go wrong if I leave the current on and how likely is it to happen.
No need to reply in english.

Henk