A subspace sandbar is an area of space where the interaction between normal space and subspace is not sufficient to create a stable warp field
The earth starship Discovery was swept into a subspace sandbar in the Beta Quadrant by a strong warp current in January 2157. The USS Baldwin was swept into the same sandbar over 200 years later, in 2377. Baldwin was able to escape the sandbar with Discovery in tow, something that Discovery could not have done on its own. (USS Baldwin: "Against the Wind")
The USS Voyager also encountered a subspace sandbar in the Delta Quadrant in 2375 (VOY: "Bride of Chaotica!")