I cannot reproduce the problem but I think that merging this fix is very reasonable because lines after source:tags/3.3.3/app/controllers/account_controller.rb@16536#L130 should be executed only when request is POST.

<pre>
$ curl -v --head http://localhost:3000/account/register
* Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 3000 (#0)
> HEAD /account/register HTTP/1.1
> Host: localhost:3000
> User-Agent: curl/7.51.0
> Accept: */*
>
< HTTP/1.1 200 OK
HTTP/1.1 200 OK
</pre>

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Hmmm, we had indeed only seen the error on Redmine 3.3, more specifically in http://www.redmine.org/projects/redmine/repository/revisions/16536/entry/tags/3.3.3/app/controllers/account_controller.rb#L148.

In current trunk, this code is now a bit different so that the exception doesn't occur anymore. However, it would still be desirable to not perform the registration from a HEAD request, as Go Maeda wrote above.

Now that I had a look around, the same issue is present in @AccountController#login@. There, it's again not an exception on HEAD but Redmine still attempts a login from the supplied URL parameters which is not desirable.
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The attached patch also fixed the additional issue described in #25653#note-2
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Patches committed, thanks.
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