I recently placed an order with homebrew.ie for some fermentation vessels, a glass carboy and a few demijohns, with these I also ordered the associated bungs and airlocks etc... to go along with the order. Unfortunately though, I ordered bungs are airlocks that are too small for the carboy (although correctly sized for the demijohns) - Disaster!
Not so however, a hawk eye watching over the orders spotted my mistake before it was packaged and shipped and fired me off an email before any damage was done to let me know the error of my ways and advised they could replace the incorrect parts with the correct parts on my order before it was shipped, and I would just pay the difference in price...
Top notch service, in my book at least.