Come on, you really check every bag of osem bisli if it has a hechsher ?
You don't?
Okay, to be precise: in my local Maayan, I check every milk product and every bottle of wine, except around Pesach when I check everything. But whenever I'm further out or in a place with different standards, yes - I check everything.
Let me ask you: how many times did you get something that was heter mechira by mistake? How many times did you get something that was made on a different run with an MK instead of a Rubin? How many products that normally have OU but had absolutely nothing? How many times - and this happened in Maayan - that they put Rav Weitzman milk on the shelf (not even Tenuva Mehadrin)?
Things like this have happened to me more than ten times. You learn your lesson eventually.