Stocking issues

So for years now I have had these issues but just can not figure out what is going on.

My stock levels seem to have a mind of their own.

I have alway not allowed checkout if a product is out of stock.

I have seen it where I have a product out of stock and the next thing you know someone checks out with one.

I have seen where I set a stock level one day and I know I do not sell those items and now they show out of stock.

I keep blaming my shipping guy for messing it up.

but I know he cant be doing it all the time.

this has been my only real complaint with OC.
I have tired all kinds of scenario to duplicate these issues but I just can't seem to figure it out.
I think maybe something is in someones cart and it lets them check out, but I can not duplicate it..

Anyone else see these issues?

Ziggy