Yes, there is a risk of CNS depression like T said.
Alcohol and Ativan can cause blackout depending on dosages and tolerance. Unusual and risky behavior can often occur with little to no memory of it. Like waking up in jail or naked in a field with no memory of what the hell happened.
There is also the risk of redosing with more Ativan and/or alcohol then originally intended. It is fairly common for people to redose when mixing these drugs from the lower inhibitions and/or from delusions of sobriety. Once redosing occurs chances of a blackout, and all the shit that comes with it, increase considerably.