When a DC line touches a AC line, then some DC Component will add to the AC line (i.e. AC waveform may rise or fall to a DC level)
It will cause AC + DC current as well.
It will cause a fault in the circuit. If there is no protective device then it will damage equipment on both AC and DC side.
The DC Component present in the resultant current saturate the cores of the AC Transformer and transformer fails.
It will cause high AC current in the DC line and damage the DC equipments, generating an enormous amount of heat and sparks.