Some of the credit-card sized, solar-powered calculators (without a battery backup), which were all the rage in the early 1990s, didn't always reset to the initial state completely when under a low-light condition, after pushing the C (clear all) button, and thus would behave unpredictably after such "incomplete reset" - I think some of the registers were not cleared due to very low power provided by the small solar panel. For this reason, it was customary to press the button multiple times - which prevented the problem.
Also, some models of the calculators had the buttons combined into one: CE/C - first push would erase the last entry, second would reset to initial state.
Image e.g. here.
