How absolute are absolutes?

When modeling a system that needs to last a long time or be used by a very large number of users - you can run into problems with absolutes. It turns out that very few aren't subject to occasional change or are otherwise subject to interpretation:

• weight - the kilogram is based on a reference original as well as forty copies. However, simply touch or dropping one can make subtle differences in weight - and in 2009 it was discovered that none of these weight exactly the same amount - each are off by a handful of micrograms. Some of these have grown steadily lighter in weight over time.
  
• length - the meter was initially defined in Paris in 1790 as the length of a pendulum with a half period of one second. In 1791 it was changed to be one millionth of the earth's merridan along a quadrant running through Paris. In 1793 the previous definition was discovered to be long by one fifth of a millimeter. Between 1795 and 1799 two reference bars were created (first brass then platinum). Then between 1889 and 2002 the definition was changed five times to consider temperature, gravity, and atmosphere, to be based on atomic decay, then be based on a the speed of light over a fraction of a second.