There was a topic about "overdonating" to shrines. It is possible to do a big donation to a shrine, even when there's only a little amount of favor needed to get an emblem. The excess of favor gets lost.
Well, today I noticed the same goes for the piggy feeder. If there's 61/80 food in your feeder and you want to top it off by, for instance, pouring in 50 grain (because as we all know you can't select a specific amount) the extra 31 grain gets lost. I filed a bug report on this, but thought I should share the knowledge.
So... pay attention when filling op your piggy feeder!