I may have got the chronology of the photocells a bit wrong here, and they may have been there from early on. The story Wes Clark tells is that the things used mirrors as part of the light path for the photosensors, and they were looking for a positive signal (light makes it past the tape, indicating that it's off or misaligned).

And apparently one of the people at Lincoln Labs was a heavy smoker, and he would rest his arm in such a way that the ash from his cigarette would fall on one of these mirrors. So when the tape slipped its moorings, the dirt on the mirror prevented the photosensors from picking up anything!

Wes Clark also tells stories (midway through the 1986 talk I posted, I think) about demoing the resilience of the #LINCTape systems to funders by tipping an ashtray over the equipment, brushing it off gently, and then loading up a tape. Apparently it got some complaints, but it proved the point that they were making something durable enough for the end user to keep their data on their person and load it up as needed.