This probably isn't good. Early this morning, a pair of Turkish F-16 fighters intercepted and shot down a Russian Sukhoi Su-24 attack fighter near the Syrian border. According to Turkish authorities, the two-seat jet was violating Turkish airspace and ignored 10 different warnings given over the course of five minutes to exit the country's airspace. Russian President Vladimir Putin called the Su-24's downing a "stab in the back that will have serious consequences for Russia's relationship with Turkey."
According to Defense News, local forces on the ground confirmed that both pilots ejected, a report corroborated by the Russian Ministry of Defense. The latter report comes from Foxtrot Alpha, and cites Reuters in claiming that one of the pilots was killed. The Su-24 was operating out of a forward operating base near the Syrian port city of Latakia.
Foxtrot Alpha reports "multiple points of view" captured the shoot down, and saw the Su-24 falling from around 20,000 feet with flames streaming behind it. Russian helicopters have been scouring the area conducting search-and-rescue operations, although FA reports that at least one of the helos has come under attack by forces from the Free Syrian Army.
Since it began operations in Syria, Russia has shown a rather flagrant disregard for Turkish airspace. Early in October, reports emerged of a couple incidents involving Turkish F-16s intercepting Russian aircraft. According to Foxtrot Alpha, Russia blamed these incidents on "navigational errors." Just a few weeks after that, Turkey downed an unidentified drone which allegedly belonged to Russia.
You can see one of the videos of the Su-24 going down in the video below. Be warned, though, some viewers might find it disturbing.