The 1972 Munich Olympics Massacre. After the failed rescue operation at a Luftwaffe base led to the death of all eleven hostages, initial reports coming out said that the operation was a success and the athletes alive and well. Then, Jim McKay's famous speech:
Jim McKay:' We just got the final word ... you know, when I was a kid, my father used to say "Our greatest hopes and our worst fears are seldom realized." Our worst fears have been realized tonight. They've now said that there were eleven hostages. Two were killed in their rooms yesterday morning, nine were killed at the airport tonight. They're all gone.
Happens often in American Football commentary. A player will hit another one really quite hard, and the commentators will hoot and holler about it, with phrases like 'Wow, what a great hit!' 'He almost took his head clean off!' till they realise that the player who was hit isn't moving, and the medics are coming on to the field. Suddenly, they start speaking quietly, with phrases such as 'of course our thoughts are with (Player X) and his family at this time'
Interestingly, soccer and rugby commentators don't go to either extreme: we get "well, that was a messy tackle", followed by "well, I hope he'll be OK" as the messy tackle's recipient gets stretchered off and "I think he deserved that" as its perpetrator gets sent off.
In the case of soccer at least that's partly because it's not a contact sport. When someone gets hit hard, it's clearly against the rules, and not a good play that happened to end badly.
Theater competitions and festivals in general are prone to this. If multiple groups chose their play independently and organizers had to set the schedule without knowing what was being performed, odd juxtapositions are going to happen.
On 6 July 2005 it was announced that London would host the 2012 Olympic games. The media and city of London celebrated. The next day three terrorist bombings in London killed 56 civilians and caused widespread panic and despair.
This being London, England, though, everybody was back using the buses and tube as soon as they reopened.
Beijing wins the 2008 Summer Olympics; weeks before they're hit with a massive earthquake that kills and displaces thousands.
Rio de Janeiro wins the 2016 Summer Olympics (and the World Cup); that winter they're hit with record rainstorms and mudslides (currently they're dealing with protests over the venues' costs and the heavy-handed way they're dealing with the favelas).
Sochi, Russia hosts an almost problem-free 2014 Winter games despite a lot of controversies; a couple weeks later all that goodwill soured when Russia "came to the aid" of their countrymen living in the Crimean peninsula with tanks and "totally not Russian" masked soldiers.
In non-Olympics sports: New Orleans is devastated by Hurricane Katrina; a couple years later the New Orleans Saints win the Super Bowl; months later a giant oil rig explodes causing almost a dozen deaths and insane levels of pollution.
Depending on which show you watch, the commercial break can be a Mood Whiplash. Have you ever watched a TV show covering an eerie subject matter (In Search Of..., Lost Tapes, etc.), and then when the commercial break starts, the first ad that comes on is light and cheerful? It can feel pretty weird.
Or the reverse: you've been watching the pleasant inanities of How I Met Your Mother and then suddenly are confronted by the image some hungry child in rags or beaten dog in a cage, as the solemn voice drones about their suffering and how "only you can help."
"Stay tuned for scenes from our next episode" can have this effect as well, depending on the show and whoever is saying it. For example, after a particularly gory and somber episode of Criminal Minds, Matthew Gray Gubler delivers the message in his usual cheerful chirp.
On the other hand, "Hi, I'm Sarah McLachlan, and I'm about to ruin your whole day."
In Finland it caused some stir when a commercial TV-station was broadcasting Full Metal Jacket, the evening sponsored by a local pizza store chain. Right after private Pyle shoots his brains to the wall the movie cuts to commercial with a happy upbeat jingle and "This delightful movie evening brough to you by..." The station later apologized about the unfortunate combination of placing the commercial break and upbeat commmercial.
Done deliberately by John Cleese at fellow Monty Python member Graham Chapman's memorial service, in which he goes from a typical recitation of what a great man the deceased was, to slating him as a freeloader who should burn in hell. Justified, according to Cleese, because Chapman "would never forgive me if I didn't."
This is nearly inevitable if you use Tumblr. It may depend on the blogs you follow, but it's very common for one's dashboard to have news about tragedies like the Trayvon Martin shooting sandwiched between lolcat pictures.
Facebook can be the same way. One friend will tell a funny story about their day or post a funny joke, the very next thing on your timeline could be "Please keep my grandfather in your prayers, he's not getting better."
The entire idea behind the BBC's "Comic Relief" charity event. Comedians join together to create a great many shows, all focused around gaining money for various charities, often to do with starving children. 2011 featured David Walliams doing a 24 hour Panel show run, and clips were shown, interspersed with David Tennant (The 10th Doctor) and others visiting Malaria-infected five year olds.
Anytime a program is interrupted by a news bulletin involving some calamity or, even worse, an "Emergency Alert Notification" of the style of the old CONELRAD and Emergency Broadcast System, which anyone over the age of 40 was conditioned to treat as a warning of imminent nuclear attack. Which is why blood pressures go up in some people whenever the same warning signal is used for amber alerts and thunderstorms.
At the 24 hour news channels, strange news segment juxtapositions cause this, whether it is due to bad editing procedures or not. In Cti news reporting of the 2012 Bhoja Air Flight 213 airplane crash was followed straight by reports of pet cafes in Japan.
Or, for that matter, any local news broadcast period will have instances of Mood Whiplash, roughly one per broadcast. For example, one news reporter will be closing up on a live broadcast from the scene of a horrific home invasion-turned-slaying and then, after a brief acknowledgment from the in-studio anchors about the news story, those same anchors will start cheerily reporting on a family fun event at a local park, say, or about the surprising benefits of some commonly consumed food item.
Happened on one episode of the Irish chat show The Late Late Show. From thousands of entries for a postal quiz, the name chosen at random turned out to be a recently bereaved mother. Straight from levity and laughter to "My daughter died last night." In fairness to him, Gay Byrne handled it beautifully.
The Christmas truce of 1914, when Allied and German forces stopped brutally killing each other for a week or so in order to sing carols, exchange gifts, and even play games of football together. Simultaneously heartwarming and heartbreaking, seeing as the fighting was back on in full force after Christmas, and such truces became more and more rare as the war went on.
A number of German soldiers were sentenced to death for refusing to open fire on the men they had just spent a week becoming friends with.