Theoretically: 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. Practically: It will cause a fault in the circuit. read more
To be specific, if an AC line touches a DC line, you will experience a short circuit with a blast and a spark on the terminals. Depending on the voltage levels of AC and DC terminals, there may be damage! I can say this because I did the same mistake once. read more