Testicle Left After Neuter

My mini australian shepherd was neutered 7 days ago and has had a pretty typical recovery. Both swelling and bruising has gone down, stitches healing well, but it appears as if a testicle has been left in the sac. At first we thought it might be swelling, but it appears all other symptoms are healing quite well. Is there another reason why it may appear this way?

Hello - and oh my goodness!   I suppose that this can happen (a testicle being left behind after neuter) but it's quite rare.  Vets tend to do a lot of these ops - there are only two testicles to remember and they can clearly be seen throughout the operation, so its unusual to forget one.  It's more common for fluid (water / blood) to leak into the scrotum afterwards and give the impression of a swollen bump or bulge, which will eventually regress on its own.  Once the testicle is removed, the blood vessel and packing tissue around it might be tied off in such a way that the can still be felt blebbing about through the scrotum, although these tend to be much smaller than a testicle.   Rarely, a hernia can pop into the scrotum.  You should have post-op checks booked for your vet to check the scrotum after the op but if something has changed quickly since then, your dog seems uncomfortable or you still have concerns, vets are usually happy to triage over the phone or do interim checks.