DISPLAYS 6.599743591E-4 - WHAT'S THAT?

Hello,
I am as surprised as you are, but not by the number. Usually people who dare to handle this calculator know something about scientific notation and higher mathematics.
No matter. What is displayed is a number cast in scientific notation the format of which is as follow:
One digit| decimal mark| 0 to 9 decimal digits| E | a - sign, or no sign| one or more digits.

The part before the E is called the mantissa.
E is a shorhand notation for times 10 to the power
The rest is the exponent. The exponent can be negative or positive and the number between 0 and 99, except for some avanced HP calculators that can display numbers up to 1E 500.

Your number is
6.599743591 x 10^(-4) or 0.0006599743591

By the way, this is a fine calculator and still the top of the line of TI calculators for engineering, mathematics and science. Take good care of it.

Hope it helps.