I updated an SSL certificate on a server used by a travel organisation. A few hours later we got an urgent call from the company saying their iOS app was failing in embarrassing ways. After investigations involving numerous parties all blaming each other it turned out that the iOS app developers, for reasons known only to themselves, had hard-coded the old certificate's serial number into their verification code. We fixed the immediate problem by reverting back to the old certificate which, luckily, was still valid for a few more weeks.