An older fuelup, #1, has an odometer (59,xxxx) which is greater than or equal to the odometer (6,xxx) that you specified for this fuelup.
The vehicle has a mechanical odometer that only goes up to 99,999.9 miles, and rolls over back to zero when it hits 100,000. To lessen confusion, I "missed" a fuelup when it rolled over, and began logging as if it were back to zero.
This must be a new implementation because it was working fine for the past 6,000 miles I was fueling up after it rolled over. I think you should have the option to override, especially in this specific case of an odometer rollover. Help!
Yes, you are correct, a recent fix was implemented to catch incorrectly input Odometers to help prevent bad data.
We've also recently added "Last Odometer: X" under the Odometer field when adding a fill-up. This is useful in the case when you're not sure you've missed a fill-up or if you think something may be off (your new odometer is WAY off compared to your Previous).
In your case, with a rolled over Odometer, I would suggest editing your past fill-ups since the roll-over and add the "1" placeholder at the beginning. Keep in mind that without making these changes, if your "new" rolled-over odometer catches up to the values input from your pre-roll-over fuelups, then Fuelly will not calculate your MPG properly.