|A piece of sensing foil from a vintage, early 70's 8-track tape|
This may sound complicated or confusing, but it's actually very simple. The risk in not replacing sensing foil is that the tape comes apart, resulting in half of the tape disappearing into the tape cartridge. The sensing foil pictured above (actual size is a little less than an inch in length) fell off of an 8-track I was playing just this morning. I then had to perform "surgery" on the tape to carefully re-connect the two ends of tape with new sensing foil (which, in case you're wondering, can be purchased relatively cheaply on eBay). 8-track surgery can be complicated and frustrating, hence why it's best to proactively replace the foil upon first play.
Thus concludes today's lesson in keeping your 8-tracks happy and healthy. I hope you found this helpful. Class dismissed!