Russia has warned the US that if it (continues to) attack Syrian military forces in Syria their planes and missiles may be shot down. 
Russian air defense system crews are unlikely to have time to determine in a ‘straight line’ the exact flight paths of missiles and then who the warheads belong to. And all the illusions of amateurs about the existence of ‘invisible’ jets will face a disappointing reality. 
This looks pretty blunt.
Since (by the US’s own admission)  any direct US military intervention in Syria would be illegal under international law – and since the Russians are there legally by invitation of the Syrian government – if the Russians were to actually shoot down a US missile or plane that would be a legal action carried out against an illegal aggression. A legal action carried out against criminals in effect.
It remains to be seen whether the US will now proceed to start bombing Syrian government forces. An alternative would be for them to supply anti-aircraft missiles to the opposition. They’ve been avoiding this so far because of the problem of seepage of weapons from ‘moderate rebels’ to radical Islamists.  A seepage of anti-aircraft weapons opens up the nightmare PR scenario for the US of an Israeli civilian plane being shot down with a US supplied missile.
One tactic the CIA uses is to supply their ‘moderate’ fighters with Eastern European weapons so as (presumably) to try to disguise their provenance.  But it seems unlikely that supplying such ‘kinetic’ weapons as effective anti-aircraft systems could be so easily disguised. And – the level of transparency in the US is such that such a covert programme would most likely be eventually disclosed.
The situation now appears to have real potential for a direct military confrontation between Russia and the US. US policy is in disarray. The danger is that the war party will seize the initiative and may come out on top.