daf bit: K'ritot 17

The mishna on today's daf discusses some cases of doubtful transgressions and brings several examples. The first set of examples is: if one isn't sure if he ate forbidden fat, or if he is sure he did but isn't sure if he ate enough to be liable, or if there was before him forbidden fat and permitted fat and he ate one but doesn't know which is which -- in all these cases he is liable for a guilt-offering because of the doubt. The g'mara spends the next two pages discussing this and casting doubt on the case of one piece of fat of unknown status. Another example where he owes a guilt-offering is if he did work and isn't sure if it was Shabbat at the time or a weekday. (17a-b)

I had thought that this mishna would shed light on the case where one eats food that, it turns out, wasn't kosher, but it doesn't seem to be going in that direction.