The HP-65 was the first magnetic card-programmable handheld calculator. Introduced by Hewlett-Packard in 1974, it featured nine storage registers and room for 100 keystroke instructions. It also included a magnetic card reader/writer. Like all Hewlett-Packard calculators of the era and most since, the HP-65 used reverse Polish notation (RPN) and a four-level automatic operand stack.
The HP-65 had a feature whereby storage register R9 was corrupted whenever the user (or program) executed trigonometric functions or performed comparison tests; this kind of issue was common in many early calculators, caused by a lack of memory due to cost, power, and/or size considerations. Since the problem was documented in the manual, it is not strictly speaking a bug.
Bill Hewlett's design requirement was that the calculator should fit in his shirt pocket. That is one reason for the tapered depth of the calculator. The magnetic program cards fed in at the thick end of the calculator under the LED display. The documentation for the programs in the calculator is very complete, including algorithms for hundreds of applications, including the solutions of differential equations, stock price estimation, statistics, and so forth.
During the 1975 Apollo-Soyuz Test Project, the HP-65 became the first programmable handheld calculator in outer space. It was carried as a backup in case of a problem with the Apollo Guidance Computer, although in the event the computer did not malfunction.