Trawling through the PGN standard, I was surprised by two of the specifications:
An escape mechanism triggered by
% at the start of a new line:
There is a special escape mechanism for PGN data. This mechanism is triggered by a percent sign character ("%") appearing in the first column of a line; the data on the rest of the line is ignored by publicly available PGN scanning software. This escape convention is intended for the private use of software developers and researchers to embed non-PGN commands and data in PGN streams.
A percent sign appearing in any other place other than the first position in a line does not trigger the escape mechanism.
A "rest of line" comment initiated by a semicolon
Comment text may appear in PGN data. There are two kinds of comments. The first kind is the "rest of line" comment; this comment type starts with a semicolon character and continues to the end of the line. [...]
They surprised me because I have never encountered them in any PGNs I've found "in the wild", so to speak. Has anyone encountered PGNs where these (escape mechanism or semicolon comments) are used?