I overstayed my visa by a few days, it wasn't much of a big deal if you overstay your visa by less than 30 days you escape paying the fine. ( You're supposed to apply for a visa extension if you know you'll overstay by a period of more than 30 days)
Should you overstay by a period of more than 30 days, it attracts a fine of around EGP 1500 ( approx US$ 100). This usually causes you to delay at the airport and if you're not careful you may end up missing your flight ( especially if you are departing from Cairo). To be on the safe side if you overstay by more than a month arrive at the airport at least 3 hours earlier to clear with migration in good time.