We have five recent tickets from donors encountering this error. #181207 uses Firefox 42.0 on a Macbook Pro, #181656 uses OS X El Capitan with Safari; waiting to hear back from #181200, 181520, 181242. It is likely that there are more of these given the understated wording with which some donors report donation errors, and that some may not contact us about it.
Description
Event Timeline
...and that this screen briefly appears as a normal part of the email link loading the form. We'll ask donors to refresh the page as an interim response.
The landing page also makes a cross-domain request to load the payment outages from meta. It's possible that when that request fails, we never get to the line of code that makes the form appear.
You /can/ trigger it by setting hpc to a negative number: https://donate.wikimedia.org/w/index.php?title=Special:FundraiserLandingPage&country=US&uselang=en&utm_medium=email&utm_source=sp50097764&utm_campaign=C15_Email1&hpc=-999
Maybe just add try/catch around the adjustments?
#181207 got this using Firefox 42.0 on a Macbook Pro; most likely link is http://links.wikimedia.mkt4477.com/ctt?kn=15&ms=NTAxNjM0MTkS1&r=MTE4ODczMDM4NjU0S0&b=0&j=ODIwNDQyNzI2S0&mt=1&rt=0 but will confirm
I couldn't reproduce what's causing this. However @Ejegg's suggestion is a good one, and I've implemented it. This should stop it from stalling on 'Loading' if there is a problem in the adjustment code.