Did a quick search and didn't see anything about this.

I had made a reservation last year with a pickup time of 8am and a return time of 4am and input my arrival flight info on UA 920 and was guaranteed a rate of 19.83 for 4 days.

In February, I modified the reservation to have a return time of 8am on the same day and visually confirmed that the pickup time was the same, the rate was the same, the 4 days was the same, and the total approximate charges were the same prior to submitting the modification. Unwittingly, I also changed my arrival info from UA 920 to UA 958. I received an email confirmation of what I had visually confirmed. Thankfully, I looked at the reservation on hertz.com and noticed that the rate on the website did not match the rate that was in my email confirmation, nor did the pickup time. Apparently, by changing the arrival info, Hertz's backend systems decided to change my pickup time to be the exact arrival time of my flight. It did this without notifying me, which increased the rental term to 5 days and also lost the guaranteed rate, which roughly doubled the cost of the car. After some back and forth with Hertz customer service, the best they could do was a rate about 20GBP greater than the original rate that I had reserved.

Be very careful when inputting or changing arrival flight information, as your reservations might silently change.