calculators like TI-84 are programmable and you can even store programs to flash. You could literally type out plain text (like formulas, instructions, etc.) and store it as a BASIC program.
A lot of teachers don't know that there is a simple way to disable programs, and a lot of students don't know about the programming functionality. They don't understand that allowing a programmable calculator is basicially making the test open-book, not to mention it allows students to solve problems computationally (e.g. numerical analysis, euler's method, etc) that they would otherwise have to solve algebraicially.