I wear monthlies but often I find myself forgetting when I open them and use them a week over.
Well it's not like the prescription is going to wear out, but the contacts can become very uncomfortable. Also, if you wear them for way too long debris can collect on them and get under your contacts and potentially scratch your eye. I had that happen once, it wasn't major or anything just a little scratch. Other than that, you should be fine. Just take em out as soon as you remember.
They may not breath as well and cause your eye to get infected, but my wife does the same thing with her's and this has yet to hppen.
I do it all the time. They will just get uncomfortable at some point and you will have to throw them out at some point.
i know its bad but when i did it a few times they started getting weird bubbles on them and got blurry a little
happen