bugs wrote:

(In reply to comment #0)

This is actually a bug of the extension DonationInterface, but it seems to be
very WMF-specific, therefor I put it in this section.

I think this is related to some JavaScript that Nimish added (=> ASSIGNEE) here: http://wikimediafoundation.org/wiki/Template:2009/Donate-now

When I clear the cache of a page on wikimediafoundation.org, the page appears
and on top of the page "Please enter a valid amount" (this is the system
message 'payflowpro_gateway-error-msg-invalid-amount').

Nope, it doesn't happen to me when purging as a logged-in user (but it does when purging as an anon). I think I've seen the same error message in other instances though, but I can't remember exactly what those were. :-(

(In reply to comment #1)

(In reply to comment #0)

This is actually a bug of the extension DonationInterface, but it seems to be
very WMF-specific, therefor I put it in this section.

I think this is related to some JavaScript that Nimish added (=> ASSIGNEE)
here: http://wikimediafoundation.org/wiki/Template:2009/Donate-now

No. It's in the page source, not added by javascript.

I haven't really looked at the code, but could it be that the it checks for action=view and $wgRequest->wasPosted() and then assumes that the user tries to use the DonationInterface? (Just wild speculations, but they would explain why the error only occurs for anonymous users)

bugs wrote:

(In reply to comment #1)

I think I've seen the same error message in other
instances though, but I can't remember exactly what those were. :-(

Today I ran into it quite frequently when previewing pages too. (as a logged-in user)

The bug was introduced in r58589 and fixed (inadvertedly?) in r58868.
I added a note about this bug on r64081.