It means the dimension is 'Not to Scale' and occurs when you change the value of a driven dimension.
Select the dimension and you will probably see that the lock symbol is off. Click the lock symbol to make the dimension a driving one and it will revert to it's true value.
Now try changing the value.
If the symbol stays off it means that you already have enough constraints to define the object.
bc.
2.4GHz Core2 Quad, 4GB RAM,
Quadro FX4600.