Air compressors compress the humidity and create water inside the compressor tank. You will end up blowing water droplets into your computer. Ideally they will evaporate right away, but that is not always the case.
This is a yes, preventable, and another example of why it is awesome to live in SoCal.
If you keep your tank drained, the amount of water should be minimal. I actually keep my vented (it has a knob at the bottom to open/close) all the time, then start the compressor (allowing the air to push out any remnants), close the vent and allow pressure to build, then vent any leftover air out the bottom.
Since it is super dry in SoCal, compressor that is about waist high will only have about a teaspoon (I was bored and measured it one time) of water. If you are in a super humid area it probably would be cups.