Here are two screenshots of my sandbox, one from my laptop and one from my phone. The page's content, at the time of the screenshots, was
{{#if:{{REVISIONID}}|If you are seeing this message, then <code><nowiki>{{#if:{{REVISIONID}}}}</nowiki></code> returns positive (and this revision's ID is {{REVISIONID}}).|If you are seeing this message, then <code><nowiki>{{#if:{{REVISIONID}}}}</nowiki></code> returns negative. Here's what happens if you type in <code><nowiki>{{REVISIONID}}</nowiki></code>: {{REVISIONID}}.}}| Laptop 'shot: | Mobile 'shot: |
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This is somewhat important, as it b0rks any modules that assume that no REVISIONID means the page is being previewed, most notably Module:Check for unknown parameters on enwiki. Here is a screenshot of the page that caused me to notice this bug:
@Reedy, adding you since we chatted briefly about this on IRC.
