A droop occurs when a small smooth portion of the seam extends below the bottom edge of the seam at the cover hook. This is near a false seam, but this type of defect can still be seen as a closed seam (i.e., it does not lose pressure). However, it can be seen visually and will ultimately cause a false seam if untreated.
Droops can also occur in some cases, by product entrapment in food (in particular fish) food cans.
This is what a drooping double seam looks like using a double seam inspection tool.
The trouble with the droop is that if you miss it visually, it is localized so just cutting in 3 places randomly may not exhibit the issue. Here is what a droop looks like on a non-destructive 360 degree scan of the thickness and length of the can – both thickness and length graphs are affected and are higher than the maximum standard specification at the same points.