If I include any of the following sequence of characters anywhere in any block in Wordpress, I get the red "Updating failed" error when I try to save the draft.

$v()

$xyz()

$x   ()

Any variety where it looks like $ followed by an identifier followed by the empty parentheses seems to trigger the problem. The following do NOT trigger the problem:

$x

$()

$  ()

Initially I thought it was because I was using a syntax highlighter plugin; I tried a different plugin and it still failed; and a post with no plugins used still exhibits the problem.

I've tried refresh the page and trying again. I've updated Wordpress to the latest version available (5.2.2).