Full-wave rectification circuit question

[K29]

Not sure if I'm posting this one in the right place because its not a "problem" per se. For the full-wave bridge rectifier we are warned not to measure the input and output voltages simultaneously with the oscilloscope! For example
http://www.sonoma.edu/users/m/marivani/es231/units/experiment_05_2.pdf"
on page 4 the warning is given.

Why is this? Does the problem arise through the earthing of the oscilloscope leads(my guess)? I can't quite picture what will happen in the circuit if you were to try to do this.

 

[hikaru1221]

I guess your input and output voltages are Vs and Vo, am I correct?
The ground clips of the two probes are interconnected and adjusted to have voltage = 0 simultaneously. So simply put it, you cannot choose 2 points of different voltages to be both ground! Here in the circuit, the two points are the lowest point & the leftmost point. Due to the voltage drop across the diode, voltages of those 2 points are different. You simple cannot put the ground clips to those 2 points, as that will result in wrong signals shown on the oscilloscope's screen.

 

[K29]

Understand now. Thanks!