After I have updated to 1.18, sometimes action=purge had no effect. I noticed this when I had a php syntax error while developing and hitting purge, nothing changed. Only when I went into page editing and preview, I got the php error.

Once, purge doesn't work on a page, I can hit purge a hundred times and it won't work.
Then I go into preview and after that, purge works again.
Looks like some caching issue, I didn't change anything in my settings and just use default caching settings.

This only happens occasionally, not very often. But I am almost sure I would have noticed this if it would have been present in my 1.17 installation since I am using purge extensively every day.
I am aware that the action=purge is removed from the url, so this is not because of just pressing F5 or anything.

Also, I have no idea how to re-create this, just waiting for the next time this is happening and I am going to notice it.
The worst thing about this is, when you are testing some stuff and can't be sure whether purge really has some effect, you might even have a flaw in your extension/template without noticing since the rendered site output still is the previous one until you edit your testing site again.

Version: 1.18.x
Severity: major
Whiteboard: aklapper-moreinfo