Some people state that ESP8266 is tolerant of 5 V logic levels on its GPIOs, while others vehemently disagree, pointing at the datasheet-stated 3.6 V maximum. Datasheets aren’t source code fo… ...

This knowledge is handy when optimizing ESP8266 designs for memory speed or maximum IO flexibility. It’s a good lesson that it always pays to look at the datasheet to get the best out of your parts.