Too much sugar with a 1 can kit can end up giving you `homebrew twang' - not exactly vinegary or sour taste, but a bit like that. Some kits seem to be worse that others for this though.
You can avoid it by adding malt extract instead of sugar but of course this then makes your 1 can kit more expensive, to the point that you might as well have bought a 2 can kit in the first place.
Another thing you can do is to `brew short' - ie. make it up to 4 gallons instead of 5. At the same time add less sugar - 500g instead of 1kg. Result is a brew that has more flavour, but about the same alcohol, and no homebrew twang. Only downside is that you end up short of 8 bottles. :lol: