A milk can lid can sit in place and look closed while still leaking or letting contamination in, if the gasket underneath it has degraded. The Can Gasket is the rubber ring that creates the actual airtight, liquid-tight seal between the lid and the can rim — it's a simple part, but it's doing the entire job of keeping milk clean and contained during transport, handling, and short-term storage.
Over time, repeated compression every time the lid is closed, along with exposure to cleaning chemicals and general wear, causes the gasket to flatten, crack, or lose its shape — at which point the lid may sit in place visually but no longer forms a proper seal. This shows up as milk seeping out during transport over uneven roads, or as a can that doesn't stay as fresh as it should because the seal isn't actually airtight. It's an inexpensive part that's worth replacing at the first sign of wear rather than waiting for a spill to force the issue.