I've used an HP-12C for 25 years. I always thought the expression, "Reverse Polish Notation" was a Polish joke because the calculation was entered backwards. Once you get used to using RPN, it's impossible to borrow someone else's calculator w/o messing up.

Wikipedia:

Friden introduced RPN to the desktop calculator market with the EC-130 in June of 1963. Hewlett-Packard (HP) engineers designed the 9100A Desktop Calculator in 1968 with RPN. This calculator popularized RPN among the scientific and engineering communities, even though early advertisements for the 9100A failed to mention RPN. The HP-35, the worlds first handheld scientific calculator, used RPN in 1972 as did the HP-10C series of calculators, including the famous financial calculator the HP-12C. When Hewlett-Packard introduced a later business calculator, the HP-19B, without RPN, feedback from financiers and others used to using the 12-C compelled them to release the HP-19BII, which gave users the option of using algebraic notation or RPN.

....RPN calculators are comparatively expensive and rare. When an RPN calculator is unavailable, frequent users of RPN calculators may find use of infix calculators difficult due to habit. The usual fault is trying to use the equals key as enter. Whatever the error, calculations get lost and have to be re-started.