Maximum Input voltage of the DSO shell oscilloscope and TL084
If you put 50V directly on the pin of the BNC connector, it would violate the absolute maximum ratings of the TL084.


  The magnitude of the input voltage must never exceed the magnitude of the
  supply voltage or 15 V, whichever is less


At that point the ESD diodes may be turning on (they don't give any details on this) or the FET spec is violated. Although the current would be low (in 10uA range) still probably not a good idea.

So why they kept it to 50V is beyond me. I would say it's 15V. 

After 63V is considered High Voltage for some regulatory standards so maybe that plays a part in it.