A Maintenance Inspector reports he and a Mechanic had signed off for a slide change at cabin door 1-right; on a B737-900; but failed to recognize the slide girt bar was too short and could not be secured in the girt bar floor fittings. A Flight Attendant noticed the discrepancy.

Narrative

While performing a task card; I witnessed the R1 door slide change. The girt bar was for a slide in the aft door positions and was too short (approximately 4 inches). The discrepancy was discovered by a Flight Attendant prior to flight and resulted in a maintenance delay. The task was performed by a Mechanic; with me as Inspector. We had in possession all applicable references and reviewed the task prior to completion. No problems were encountered. While accomplishing step C.(1)(d) of Maintenance Manual 25-66-01; he connected the girt bar to the forward retainer verifying its operation; disconnected it; and then connected the aft end to its retainer separately. He and I were focused on the individual retainers rather than the assembly as a whole; and never realized that it was shifting side-to-side. He indicated to me that there were no issues and I saw no problems. He is a fairly large person and filled the R1 door area; partially blocking the view. The only abnormality was that the slide pack was received with the girt bar already installed. Normally; there is no girt bar and we transfer the one from the old slide. The slide was serviceable and it never occurred to us to inspect the girt bar itself. Until this incident; it never really occurred to us that they were different lengths; having swapped rather than use one already installed from Stores.