There can be several reasons for this: The sprayhead wasn't fully depressed. Too much pressure was applied to the front third of the sprayhead, causing the spray jet to point downwards and come into contact with the edge of the spray can, where drops can form. As they accumulate they are spattered onto the paintwork.  The spray can was held horizontally for too long. As a result, the internal dip tube has drawn in more propellant gas than paint, resulting in paint spatters / irregular spray patterns. The spray can was too cold. To attain optimum atomisation, the temperature of the can must be around 20°C.