I was editing an existin post inline and when I saved the post it threw an error: let's try to replicate it here:
Here's a post saved first to this point without "bang equals" in it.
The Initial post was saved without error, this line was added in an inline edit box.
After saving, I added this line with != in it.
Hmmm. no error: let's try "right!=legal"
Hmmm. no error again... maybe just a system hiccup.