I was running 700 watts working some DX on 40 in the evening when Vermont decided to deliver rain and a 1/4 inch ice storm.  In the morning, the driveway was unusable and so was the G5RV which went dead.  The balun was shorted on the input side.  Whether it was high power, water and ice, both, or the RF devil, who knows?  That said, here is what the inventor of the G5RV, Louis Varney, had to say about using a balun with the G5RV antenna:

It is now known that if a balun is connected to a reactive load presenting a SWR of more than about 2:1, its internal losses increase, resulting in heating of the windings and saturation of its core (if used). In extreme cases, with relatively high power operation, the heat generated due to the power dissipated in the device can cause it to burn out as well as the PL jacks. However, the main reason for not employing a balun in the case of the G5RV antenna is that, unlike an antenna tuner which employs a tuned circuit, the balun cannot compensate for the reactive load condition presented to it by the antenna on most of the HF bands, whereas a suitable type of antenna tuner can do this most effectively and efficiently.  (Construction guide of the G5RV (II).

After reading what Varney has to say about baluns and what other hams on 40 cw had to say about them, off came the balun and on went a 9-1/2 turn home made RF choke made out of coax cable (to keep the shield from radiating) and a Folger's coffee can.  The G5RV is back in business and better than ever.  The images below can be clicked for larger versions.