If you pause a an activity, move to a new location before resuming it is possible that the recording device will calculate a speed between the two points. Or this could happen if GPS lock is lost (quite possible with the pause when not moving) and you move to a new location before it re-establishes.
For example:
You stop and pause the activity, GPS lock is lost, you walk two hundred meters down road, resume activity, the GPS has not re-established yet the time starts and then Gets the location, the device sees that you have moved a distance in a very short time period, hence a high speed.
