Many people know this, so call it a public service announcement or something.
In addition to the 'objects in the floor' bug (see the Bugs Forum and Hot Issues for more details - please don't derail this topic with that one, but Bob is welcome to hijack any thread he pleases)... there is a bug related to objects that fall or are dropped not appearing at all.
Common places you'll run into this.... batterflies releasing guano, pigs and plop, pork fountains, and rearranging objects/sorting. On a rare occasion, it winds up being something weird like an empty slot in your inventory or bag after you've collected, picked up, or been given something. My most frequent run-ins of this type have been when receiving critters after a chicken has finished incubating eggs.
I'm pretty sure it's the awesomeness of Flash causing it, but it's almost always associated with something 'appearing' in the game window. Reloading the page generally fixes it. Pay attention to what critters are saying when you feed them. If you want to be especially cautious, make sure to reload after doing something like a pork fountain.