Bug T115171 let us record the wrong amount in certain cases where a donor had payment forms open with different amounts in multiple windows. A donor on 10/6 alerted us to this, and I found one more that day. That was 2 out of almost 5,000 donations, so they're likely not very common.
@awight suggested a general-purpose log parser interface that takes a callback, and calls the function with all the info from each donation found in the log.
The bug likely affected GlobalCollect, Worldpay, and AstroPay donors, but we should check the other gateways to be sure.