A pocket corner often needs a small reinforcement where the pocket opening joins the trousers. Look first for the original stitches pulling free. If the outer fabric has torn into a V or the pocket bag itself has split, repair that damage separately before reattaching the corner.
Separate the pocket bag from the outer leg
Empty the pocket and turn the trousers inside out. Follow the pocket bag to the top and bottom of the opening. Compare the damaged corner with the other pocket. Mark the original contact point with chalk. Slip a finger or card into the pocket while you position the layers; the aim is to catch only the layers the factory stitching held, leaving the pocket open.
Add support if the anchor is weakened
On firm fabric, you can restitch the original attachment by hand using small backstitches or a small cluster of reinforcing stitches. If the cloth has thinned around the old stitch marks but there is sound fabric farther out, baste a small matching backing on the inside. It should lie flat and extend into firm material. Reinforce where the corner meets the backing. Do not pack many tight stitches into a frayed spot, as those stitches may pull through together.
Keep the opening aligned
Start on the sound side of the join. Attach the corner to its original position without pulling the pocket mouth up or narrowing it. Check the position from the outside after a few stitches. Secure the thread on the inside and remove any temporary card or basting. If the pocket is built into a lined or complicated seam, stop rather than guessing which layers to pierce.
Try the pocket with a light load
Slide your hand in and out and look for a new crease radiating from the corner. Carry a small everyday item briefly and check the stitches do not shift. If the pocket bag is pulling apart further down, mend that opening before using the pocket normally.