I've been working on the JSON RPC framework, which was previously public, but has gone private.
During its development, I've discovered at least 2 bugs in the Delphi RTL regarding UInt64:
Despite the System.JSON framework being in the RTL for several years, it seems it's still not mature enough, because when you need to parse the interpretation of a number (or in the case of the above bugs), you have to rewrite nearly the entire class for TJsonTextReader, and hence, a proposal was filed to update the class.
Hopefully, the Delphi team will fix the issue in the next release of Delphi.
In 2017, with the release of Delphi 10.2 Tokyo, Embarcadero introduced a specialized implementation of the Observer pattern into the System.Classes unit. While it has been in the wild for 9 years, it remains a "hidden" architecture for many, primarily because it serves as the invisible engine behind LiveBindings. Other than live bindings, you can also use the Observer pattern as a way to update component settings to the Windows registry, an .ini file, or persist it elsewhere.
System.Classes