How to remove an unslotted length-2 technic axle from a one-way axle hole?
The method I use is not as "purist" as Wrzlprmft's, but it also works. Wrap the exposed section of the axle in a piece of cloth (a T-shirt works well), then gently grab the cloth-wrapped axle with pliers and pull out. The cloth is to prevent any marring of the axle piece, and it also provides a good grip for the plyers. Needle-nose or other small pliers work the best as they give you the most control for manipulating small LEGO pieces.