I have been curious about 'too' meaning.
In this sentence "You cannot be too careful.", does 'too' have the meaning of 'limited extent'?
In this case, "too" means "excessively".
No matter how careful you are, it's not more careful than you should be.
You can never be too careful
you can always be more careful than you are now
has the meaning one can never be overly cautious, just as
You can never be too rich*
you can always want more money
"too" has the meaning of "too much" or "overly".
*I refrain from citing the other half of this usual idiomatic phrase