<p>When I was a teenager, I wrote a <a href="https://hachyderm.io/tags/Forth" class="mention hashtag" rel="tag">#<span>Forth</span></a> compiler, running on the BBC Micros we had at school. It had a code generation bug, which I didn&#39;t find until 20 years later.</p><p>It was very much in &quot;Baby&#39;s First Compiler&quot; style: completely trivial unoptimised code generation, as close as possible to just reading the source file and emitting a fixed fragment of machine code per word, with only control structures like IF/THEN and function definitions requiring anything more complicated.</p><p>I&#39;d chosen Forth in the first place, of course, for the same reason: it requires the least complicated parsing, so it&#39;s suitable for a first compiler. I had _read_ about LR and recursive descent parsing, but didn&#39;t fancy attempting either one in 6502 machine code. (Not even if I used lex and yacc on some other machine to generate automata and imported them on to the Beeb.)</p><p>So how did a compiler that simple manage to have a bug?</p><p>(1/2)</p>