A character’s head touches the edge of a window behind it, and suddenly the window seems attached to the hairstyle. Nothing is wrong with either object in isolation. Their outlines have met in a way that makes the spatial relationship ambiguous.
Illustrator James Gurney calls this a tangency: shapes touch without a clear overlap. His discussion of tangencies in observed scenes explains how they can attract unwanted attention and confuse depth. The same visual problem can occur in a manga panel, an animation layout or a photograph.
Try two tiny corrections
Draw a simple figure in front of a rectangular window. In the first version, let the top of the head just touch the window edge. In the second, move the window enough to leave a visible gap. In the third, let the head clearly overlap the frame.
Compare the three at the size the panel will be read. The gap separates the shapes; the overlap makes their order clearer. A tiny adjustment can communicate more than adding detailed shading to an ambiguous join.
Repeat the exercise with a hand touching a speech balloon edge or a prop almost touching the panel border. Check whether the contact creates a connection you actually intend.
Do not turn a useful check into a universal ban
Gurney’s earlier discussion of tangencies in composition also considers deliberate flattening. An artist may want shapes to connect into a decorative pattern. The question is whether the contact supports the image, not whether every outline follows a prohibition.
When reviewing your own page, temporarily hide texture and effects if the drawing setup allows it. Look at the main silhouettes and the edges around faces, hands and important objects. Make a small change, then compare it with the original rather than automatically moving everything apart.
The goal is readable intention. If two objects should occupy different depths, show that relationship clearly. If their contact is a deliberate graphic choice, let it remain for a reason you can explain.