Generally speaking, laws are made based on principle of both certainty and flexibility. It is expected that the binding forces of laws be applied to all situations in a same way . However, in real life situation, there are numerous cases whose uniqueness require the laws to be exercised with flexibility so that justice is best done. As far as I am concerned, laws should be flexible enough to cater for the specific circumstances and accomodate itself with the ever-changing social environment.
It is true that stability is an essential characteristic of laws. Consistency and predictablity are so important because without these traits, the public would be under great apprehension of uncertainty. It will affect people's decision making if they are unsure about what kind of behavior are deemed as illegal, what is conferred to them as their rights protected by the laws and what kind of remedy they can seek if their rights are infringed.  In this sense, law is like a ruler. You can never measure things with a elastic ruler.
However, it would be presumptious to equate stability with invariablility. The very quality of uniqueness of each individual cases requires that in order to be enforced with justice properly done, laws should take account of different situation that it is facing. The best example to this is minority autonomy. In China, there are five provinces where the majority of the population are made of ethnic minorities. These autonomous zones enjoy a great level of freedom in adapting laws to the local tradition. For example,  polygamy as a marital practice is generally deemed illegal, but is allowed in some minority groups, such as Yi, because it has been a normal part of their life. In allowing certain flexibility in laws, the uniqueness of the local culture and accepted practices is respected.
Moreover, the ever-changing social environment also demands that laws be adaptable. With the explosion of inventions that comes along with the information technology revolution, the society is changing so quickly that sometimes we discover that past experience become inadequate in helping us figure out how to act in a new situation. Laws, unfortunately, partly fall under this category. When new questions pop up, inflexible laws will put themselves in a difficult situation because they are too limited to do the justice right. What happened to Tencent, the Chinese counterpart of MSN, best exemplifies the dilemma of laws and the agony that the difficult situation is causing to those people involved. This company received a request from Mrs. Wong to gain access to the account of her husband, who just died in a car accident, so that she could collect some precious dialogue and picture record between them as a memory of their marriage. Even though the company feels very sorry for this unfortunate young lady, they could not fulfill her wish because according to existing laws, virtual properties are not subject to the right of inheritance. This is just one of the numerous cases in which the development of technology is posing challeges to existing laws and demands more flexibility.
To sum up, while being stable and predictable in general, laws should be made with considerable level of flexibilty and adaptablity so as to take account of various situations, both due to the intrinsic uniqueness of various social phenomenon and due to the challenge of new problems posed by the technology development. Only in this way can the justice be done in the best proper way we could.