RFC #…# can be seen as a template with one parameter, but with a special syntax just for this application. If you do not want this behaviour, put something like
<nowiki>RFC</nowiki> in your wikitext.
RFC must be spelt all upper case, and alternate names such as
FYI cannot be used, even if they exist. There must be no other punctuation but one or more spaces (ASCII: 0x20, SP) betweeen the characters
RFC and the number. The number is terminated by the first non-numeral. Hence another way to prevent the magic link from happening is to write RFC 123 which generates RFC 123.
Leading zeroes making up for a total of four digits can be used in the RFC number, which may come handy in lists and tables. Yet beware: they work only, because the IETF does support such URLs on their website.
Using incorrect numbers, e.g. of non-existing RFCs, will display links that lead to an error page as long as the corresponding pages have not been created on the target server.
See also: Markup spec/BNF/Magic links#RFCs