[Vwdiesel] glow plugs

[Pam & Doug Boes]

Is it common for all four glow plugs to fail at about the same time? If 
one or more glow plugs are shot, do the remaining plugs each get more 
current because they are connected in a paralel circuit? If this is the 
case, does the failure of one quickly cause the failure of the rest from 
over current (overheating)?
Can cycling the glow plugs on especially cold days cause them to fry 
faster?.
Normally, I go to the block heater when the overnight temperature goes 
below 10 F. Last night, my daughter forgot to plug hers in and the temp 
went to 4 F this morning. No Start! I got it running this afternoon but 
later on it would not start again when the temp had improved to around 
20F. I jumped it and had plenty of cranking but not even a wimper of 
firing. When I finally got it home around 8 this evening, I checked the 
fuse (only a 10 A on an 85 Golf?) and it was fine. I pulled the glow 
plug from #4 because it is the only easy one to get out and found it 
failed the ohm meter test for continuity. I'm sure the other three are 
the same way. I just had all of these  glow plugs out at the end of the 
Summer and they were all good.
What would cause all the plugs to fail so quickly?