Time Travel and Currency
Borges has a story where as an old professor, he meets his youthful self on a bench (in Boston?). The older version tries to convince the younger one about the passage of time and the anachronicity of their meeting by showing a dollar bill and pointing to the date (year of print) on it that would prove to be in the future for the young Borges. A minor mistake since US currency unlike the coins did not have date on them in the past (now they have the series year). Other currencies -- Euro, British pounds -- also do not have a year/date on them it seems from my preliminary test.