Robbing can be very serious. If it gets started it is hard to stop. I had a beekeeper rent an extractor today to extract his honey crop. When he went to pull his honey he found his colonies dead and empty. The two hives were overcome by bees, wasps, and hornets. The frenzy starts and it doesn't stop until there is nothing left.
The beekeeper found his bottom board covered with dead bees and capping wax from the honey frames that are now all empty.
How does this happen? The most common way is beekeepers put their wet extracted supers out by their hives for the bees to clean up. What can happen is the bees finish with the supers, then turn on the first hive in the beeyard. A whole beeyard can get wiped out by this robbing.
Preventing this can be as simple as putting in entrance reducers now with the largest opening open, If it appears robbing is happening, closing to the smaller opening can help, sometime the entrances have to be closed to stop the feeding frenzy.
If a beekeeper has wet supers to clean up, it works best to put the supers on top of the inner cover and put the telescoping cover on top of the supers. Make sure the cover is on tight with a brick on it. This keeps the bees from sneaking in the top. The supers should be picked clean in three or four days, They can be removed an properly stored.