Your terminal converts visible text into control statements? Which terminal is that? Is it supposed to do that?

Mac Terminal does at least.

Well, I can't really say I'm surprised that Apple would to do something like this.

If you type this:

$ echo '##asdf##'

...what does it print?

I can't reproduce this in Terminal.app.

The echo thing doesn't repro, but I do see the error when running arc help --full

Does this fix it?

diff --git a/src/workflow/ArcanistFlagWorkflow.php b/src/workflow/ArcanistFlagWorkflow.php
index d842072..b00fd28 100644
--- a/src/workflow/ArcanistFlagWorkflow.php
+++ b/src/workflow/ArcanistFlagWorkflow.php
@@ -40,7 +40,7 @@ EOTEXT
   public function getCommandHelp() {
     return phutil_console_format(<<<EOTEXT
           In the first form, list objects you've flagged. You can provide the
-          names of one or more objects (Maniphest tasks T#\##, Differential
+          names of one or more objects (Maniphest tasks T#\\##, Differential
           revisions D###, Diffusion references rXXX???, or PHIDs PHID-XXX-???)
           to print only flags for those objects.

not really

Hey, it does that for me too (putty) -
arc help flag looks right, but arc help --full and scroll up does do the background flip thing (arc help --full --ansi | less -r too).

I take back what I said about Apple before.

In phutil_console_format(), we convert ##xyz## to inverted text, but the \ present in the input is intended to prevent this.

It does prevent it on my machine, so presumably this is some weird ambiguity with preg_match() rules between PHP or PCRE versions. Since I can't reproduce this, I'm not sure how to fix it.

If the \\ fix above doesn't resolve it, the fix is probably fiddling with the regexps in PhutilConsoleFormatter::formatString(), but I'm not sure how they're misfiring since they work correctly on my machine and I don't see anything ambiguous in the construction.

Sry for the long waitingtime !
As @chad say the echo test did all right.

I just did a quick diff to fix (avoid) the glitch.