This piece below in quotation marks is taken directly from a comment from Teja on the Google Groups page HERE
Currently the Mr Beam machines use soft limits. They are programmed with a work area and the machine/laser head moves within this area. Occasionally the outer frame will be hit but the software recognises this and stops the machine head in a moment to avoid damage.
Additional endstop switches can be added to make the machine stop when the installed button is depressed.
“If you want to switch to hard limits, you need to do the following steps: 1. attach the switches at the plastic parts that they are triggered at the positive axes' ends 2. wire them in parallel to the existing switches 3. switch soft limits off in grbl by sending a $20=0 4. switch hard limits on by sending $21=1
For perfect working hard limits you will need to replace the endstop cables with shielded ones. The Arduino interrupts are super sensitive to cellphone and other electrical radiation.
As long as you don't need endstop switches on both axis ends, I would recommend to stick with the soft limit solution.