A classic example of a massive and infrequent event would be the person who fortuitously has a video recorder handy when a tornado hits. An example of understanding slow continuous changes involves the corn to orange case mentioned above. In 1990 I happened to see a field in eastern Mexico that had orange saplings planted between the rows of corn. As the trees matured and the canopy cover increased, there would be less sunlight for the corn. A keen eye and special observation skills are certainly helpful for geographers when it comes to measuring such changes.
Controled observation is much more accurate than incidental observation.
It is also more expensive and time-consuming, and requires greater training, but it involves fewer powers of observation. Typically sample plots or test areas are monitored closely over a lengthy period of time. An example of this might be a series of erosion pins used to quanify the amount and rate of soil loss. Another might be to record business turn-overs along a particular street. New things can be observed.