heh, nothing is angry. When you mark a fill as
partial, Fuelly skips calculating the MPG for that tank
and the next one. Fuelly needs a baseline full tank to start calculating MPG for individual fuel-ups again. But all of fuel-up data goes into calculating the average MPG of the vehicle, so it's still worthwhile to include partials.
There's a little more info in the FAQ:
How do I account for partial fuel-ups?