Just being cath'd at all can make you swollen enough inside you're urethra that it's hard to pee for a day or two and some women have bruising there too from the urethra being squished between the baby's head and the pubic bone.

Usually once the bladder is emptied it's "crisis averted" but in the rare cases when it ruptures it really IS a crisis. Ina May Gaskin says, in Spiritual Midwifery (Fourth Edition) "Make sure that her bladder is empty before the mother starts pushing. This will make room for the baby's head to pass through. Sometimes the mother won't have the urge to pee and you will need to remind her. Emptying the bladder will also prevent its possible laceration in the second stage. If the mother can't empty her bladder and it is full, let your doctor know. She may need to be catheterised. The mothers lower abdomen should never look like this (Fig. 56)" Figure 56 shows the bulge caused by a full bladder. I think basically when you can see it like that it is a dangerous situation.