An employee was replacing bolts on the lumber stacker. The employee grabbed a ladder and climbed up under the stacker between the carriage and an I-beam to replace a bolt. After the employee finished, he instructed the operator to release the e-stop so he could watch the stacker run. However, the lumber stacker was pre-cycled and once the e-stop was disengaged the carriage pinned the employee against the I-beam. The employee sustained fractures to their chest and head, a concussion, and internal bleeding. The employee was hospitalized.