Placebo Effect Reactions
Few examples in which placebo looks effective even if it isn't (Wikipedia):


The condition, for example, common cold heals on its own in participants who received a real drug and in those who receieved a placebo.
In psychological and other conditions in which the participants are asked only how they feel after the treatment, some of them may just think that they feel better.
Sometimes neither the real drug nor the placebo work, but the condition has a naturally fluctuating course, so it looks that they both work.