Not "this" story. Everything above has happened on several projects. The cat thing comes up because it is tricky to expand two macros next to each other without whitespace. So if you do:

  define(`foo',`hello')
  define(`bar',`world')
  foo bar
  foobar

You will get:

  hello world
  foobar

Working around this gets tricky, so someone inevitably ends up writing a cat-like macro such that you can do

  cat(foo,bar)

To get

  helloworld.

A side effect of this is that now "cat" is really "cat()" which expands to "". You can work around this by doing `cat'. However, if `cat' is used as an argument to another macro (such as a for loop), the quotation only prevents escaping the first time. When the for macro is expanded, the quotation marks are stripped, giving you just "cat", which gets expanded again. A correctly written for macro would add new quotes as needed, but I have never seen someone correctly write such a macro without just copying it.

Not sure if I have seen this interaction specifically with for and cat, but I have seen an interaction like it on almost every project that used m4.