There are 3 labelled boxes, but each of the labels is on the wrong box.
One box contains just apples, one box contains just bananas and the other contains both apples and bananas.
By just selecting one piece of fruit from one box can you determine the correct labels?
Take a piece of fruit from the box labelled apples and bananas.
If an apple is taken, then this box is the apples box (as it can't be the apples/bananas box as all of the boxes are mislabelled). This then means that the bananas box is labelled apples, and the apples/bananas box is labelled bananas. Similarly, if a banana is taken, this box is the banana box, the apples box is labelled bananas and the apples/bananas box is labelled apples.