A week and a half ago I discovered that my scales weren't giving me an accurate reading because a cable had gotten stuck under there (yes, that part of the house is a mess, and will be for a couple more months). I nearly cried when I realised that I'd only lost 1 kilo instead of the 2 I was so happy about. I had a bit of a slump, where I felt like it didn't matter what I did and I was supposed to have reached that goal by now, blah blah blah.
But I got back on the bandwagon after a few days, and I'm seeing good results again. My 7-day average looks a bit flaky but a few more days will put it back on track again.
re: exercise bumping up your calorie allowance - I'm using LoseIt! which keeps your allowance at the same level until you meet your goal, but subtracts from calories eaten for that day if you do exercise. It makes more sense to me that way, because you've worked off a portion of what you've eaten. And because it gives you a weekly graph as well, I treat being under my daily amount as a way of offsetting the days when I miscalculate and end up a little bit over.