Summarize this article:

Compared to a larger hole, a split seam is fairly easy to deal with. This means: where two pieces of fabric were joined by stitching, the thread is broken or missing and the seam is now open, creating a "hole." In most cases, you can simply sew the seam back together. This can be inside a pocket, causing all your change to drop through. It may also appear in a sleeve seam, causing your elbow to stick out.

Summary:
Identify a split seam.