Bloom development starts in the bud phase of the plant. Buds are forming weeks before they enlarge and open into flower. In the period that the buds are forming, extreme weather can impact the outcome of the first few blooms. This can include extremes in heat, water or the shock of a heavy fertilizer application.
By the time your flowers open, the weather is seasonally moderate, but some of the early flowers may show the stress the plant was under during bud formation. Much of this is out of the hands of the gardener, but it is good to understand it.
Most common is a greenish or speckled green edge to the flowers. Some clematis lovers appreciate these early blooms, which are more common on white and pastel clematis, but can occur even on red clematis. Other unusual early blooms may be smaller or larger than the typical blooms and occasionally you might even see some that would be called deformed. Deadhead these from the plant as soon as you notice them so that it can get on with it’s normal bloom production.