An excellent example of an evolving conspiracy theory is the debate over Chemtrails. If you start watching the sky carefully, you will notice that sometimes there are trails left behind planes such as in the picture above. Under certain conditions contrails form behind jet aircraft. These conditions usually involve planes flying at very high altitudes (above 8km) and very cold temperatures (less than -40 degrees Celsius), but contrails can form in certain other conditions.
Chemtrail theorists suspect that these clouds are evidence of an intentional spraying program. One common idea is that chemtrails are a secret program to combat global warming by spraying reflective particles into the atmosphere. Anyone who watches the sky on days when these clouds are sticking will notice that as time progresses the clouds tend to disperse and eventually "grey-out" large areas of sky.
Even if these clouds are just "harmless" aircraft contrails, the fact that they can completely block out the sky seems like a major environmental hazard. This likely helps to explain why chemtrails are such a popular conspiracy topic. There are numerous time lapse sequences available on the Internet that demonstrate the thick cloud banks being formed from the jet contrails/chemtrails.
The meme of chemtrails invites a large following because people can observe these clouds for themselves. Since there is something going on, it is easy to project fears onto this phenomenon. If regular airplane traffic is able to blot out the sky, shouldn't there be more regulation regarding flight patterns or at least some official discussion of the environmental issues? The dismissive response to this point by government and military officials makes it easier to imagine vast conspiracies behind the observations. Have you noticed the chemtrails? What do you think?