This is a failure of the bladder muscle to contract. It can occur in situations such as bladder outflow obstruction in BPH. Patients who undergo relief of obstruction (e.g. by TURP) may still not be able to empty their bladder fully due to this weakness of the bladder muscle.
This is an anatomical variation that you may be born with. It usually implies a doubling of the urinary system, commonly giving rise to 2 ureters instead of 1 on each side. The duplex system may be on the left or right side or, on rare occasions, both sides.