We've all been there. Made a dish that doesn't meet our expectations or is just plain screwed up. It happens, and as you can see by the numerous responses, just being a professional chef does not make one immune. The goal is to learn from your mistakes-figure out what went wrong and how to avoid that problem the next time. That's my general response to the OP. To get a little more specific with the mashed potato issue, below is my guide to making the best mashed potatoes (comments, contradictions, etc. welcomed).
Pete's Guide to the Best Mashed Potatoes
-First off, I admit, I use Russets (Idahos). I know that many people out there prefer Yukons, but I like Russets.
-Peel and chop into 3/4 inch chunks and place in heavily salted, room temperature water.
-Bring to a boil and reduce to a hard simmer/slow boil and cook just until the potatoes are tender.
-Drain in a wide colander and allow the steam to come off the potatoes for about 3-5 minutes (the more moisture you allow to escape the more fats, in the form of butter and cream you can add)
-Using a heavy whisk or potato masher mash the potatoes until no chunks remain (as you as you don't add any dairy at this point you'll be surprised how long you can mash them without them turning gummy)
-Season with salt and pepper (use white pepper if black specks bother you-personally I prefer the taste of black pepper so that is what I use)
-Add butter, softened but not melted. (I probably use about 4 tablespoons per pound of potatoes-this is what makes them so rich)
-Heat either half and half or heavy cream and add just enough to get to the consistency you like (forget milk, and definitely never use reduced fat milks if you want great mashed potatoes)
-Once you add the butter stir as little as possible to avoid gummy potatoes