Which models of HP calculator are the most "error" free? I would categorize "bugs" (like the cosine bugs in the 33s and 35s) or "compromises" (like the Woodstock y^x implementation using logarithms "2^3=8.000000002") as "errors". Additionally, program execution errors, where the fault lies in the calculator, and other operational glitches ("1 1 . <- ENTER" bug in some 11c's) would count. I would not count the practical limits based on the number of digits of precision a calculator has and any unavoidable rounding that this would cause. Thus, a good 10-digit calculator and a good 12-digit calculator would score equally. I would also exclude any keyboard, switch or battery problems, as they are mechanical or electrical rather than algorithmic in nature, unless they were so pervasive as to make the model unsuitable for general use.
Models with errors that I have experienced or seen reported: 35, 21, 25(c), 11c(at least some of them), 15c, 32sii, 33s, 35s, 42s, 50g. What about the Spice series or the 32s? I have not heard of any errors in those.