tank overflow, nope.
Good question jetflee, but that is not a problem if you do it right. The idea of an overflow, whether a box or a drilled hole in the main tank, is that if the sump's pump fails, water will drain out of the tank until the level goes below the overflow. Then, on the sump end, the tank is usually only 1/3 or 1/2 full, leaving plenty of space for the extra water to fill in from the main tank in case of a power failure. This is probably the most genius idea in the hobby, besides the protein skimmer and refugium filters.