A tweet appeared on my twitter feed yesterday urging people to buy loose fruit and veg to avoid packaging waste. However loose veg leads to 20% higher wastage than packaged veg, so while you might save on a plastic bag (you're going to need a container to get them home anyway, even a paper bag has an impact), you're going to be responsible for 20% more land-use, 20% more irrigation and fertilisation, 20% more washing and processing, 20% more transport and 20% more waste. I haven't done the sums, but I'm guessing the packaged fruit comes out on top by a country mile. Excess packaging is wrong of course, but we package goods for a very good reason.
You get similar simplistic thinking about bottled water. Now I try to remember to take tap water out with us on family trips (Mrs K is much better at this than me), but if I don't have any and I have to buy a drink from a shop, which is more eco-friendly – bottled water or a soft drink (= bottled water + sugar + chemicals)? I've seen people buy a coke rather than water on this basis - madness.
This simplistic good/evil demarcation in the environmental world is potentially damaging. The anti-nuclear move in Germany has propped up the coal-fired power sector. As Mark Lynas points out, the vilification of carbon offsetting by green commentators has almost certainly had a negative impact by cutting off a flow of finance into green projects.
These issues aren't particularly complicated but the dogmatic mantras of some campaigners can do more harm than good. Let's think before acting.