...Measuring our kitchen incorrectly. Sounds dumb, and all we could have used was a triple check on the numbers. My husband, the measurer, said 1/4 in. and I, the recorder of info, heard 1/4 ft....or something like that. The details are long behind us! The mistake meant ordering too much countertop, but the solution was to cut the extra piece (we bought wood counters) to use as a cutting board. The mistake also meant that our fridge didn't fit as planned and we couldn't have the extra set of cabinets we'd hoped for (after renting a truck and making an IKEA pickup, we had to turn around and return a set). All in all, we worked around our deficit but the hassles were far from pretty and ended up costing us some time and money.
OK, that's only one of our examples. What about you??
Photo: Laurel and Hardy from The Alternative Film Guide