PL/I. The argument there was that one cannot assume anybody to remember all the keywords (over 300 in some implementations. See https://www.cs.vu.nl/grammarware/browsable/os-pli-v2r3/), so they weren’t any reserved words, allowing, for example

    IF IF = ELSE THEN IF = ELSE - 1; ELSE ELSE = IF + 1;

Writing a compiler for that must be fun, especially if one wants to produce useful error messages.