How about when demand is very low.
North Carolina's Turnpike Authority was set to raise the toll on the Triangle Expressway (they've since delayed doing so), possibly a good idea for raising revenue if demand is inealstic or for reducing use if it's overcrowded. However, the toll road is basically empty (as seen in the video below), so raising the toll when you have such excess capacity won't lead to increased revenue. On about a five mile stretch, I typically see no more than one or two cars traveling in either direction during the duration of my drive on the toll road.
Also, the Turnpike Authority spokesperson noted $1.54 million in revenue from tolls last month. The present value of $1.54 million/month, assuming a 1/2 percent per month discount rate, is about $308 million, not even 1/3 the cost to build the toll road.