I don't think I have ever come across a negative pledge agreement, but I have seen multi-million mortgages on property that isn't worth half a million.  In fact, it is relatively common up here for a bank to cross-collateralize commercial loans with residential property. 

For example, a small business owner needs a large line of credit but he doesn't have enough equity to cover it, the bank will sometimes try to securitize it with any property the owner might own.  Sometimes, the business is in a different county and they use separate instruments.  The result is a mortgage for a couple of million dollars on a home worth about $350,000 - its usually a second or third mortgage to boot.

The bank is just trying to get as much security as they can.  I think they know that there isn't really much there, but something is better than nothing.

I have also seen commercial loans that kind of do the same thing with several commercial locations spread out across the state, or country. They have one huge note for several million and they file separate mortgages in each county showing the full amount of the note.  There is sometimes no reference to the other mortgages, or properties, so it looks like an insanely large mortgage on one property. 

Best,
Robert A. Franco
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