Ghost comma inserted in pattern when grout was expected

[disconcision]

david I wasn't sure if this was a bug or intended (I can see both ways), but I think you implied it's a bug so I'm filing it. If not a bug I definitely have questions about the level of smartness involved.

Given:

Pressing space yields:

This occurs in many other situations, where it is perhaps more questionable; the above is a minified example.