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This has been bugging me for a while. A party is traveling on a glacier and adds braking knots evenly to the length of the rope as seen in this answer. The person leading the rope falls into a crevasse and the knots do not stop the fall.
How does the rest of the team pull the leader out given that the knots would jam up the system?
In a 3:1 (Z-pulley) haul, the victim's rope is used for hauling directly. As you point out correctly, a surface rescue is impossible if you have knots in the rope, since the rope is under tension and you cannot untie the knots.
However, you can also drop a different strand of rope down to the victim and haul them out with that (it's then called a rescue strand). Since it's not under tension, it's possible to remove all knots from the rope before sending it down.
Some examples are a 2:1 (C-pulley or drop loop) haul or a 6:1 (C cross Z) haul, where in both cases you drop a bight of rope down to the victim. They attach the rope (with a pulley) to their harness, and then the rescuers haul. You could also send down a single strand of rope down and do a 3:1.
In any case, for a surface rescue, you need some rope available on the surface. All climbers tied together on a glacier should have some additional rope to perform a rescue.
Do note that all these require the victim to be conscious and able to help. If this is not the case, a rescuer needs to rappel into the crevasse, potentially address any immediate life-threatening injuries, clip the rope to the victim, ascend back out, then haul. This gets quite complicated, but is part of the guides' exam.
The last option would be for the victim to self-rescue, passing their prussiks over each knot in the rope. Clipping into the knots is possible and makes life a little bit easier.
In addition to Felix's excellent answer of what should be done it practice, it is well worth knowing how to pass a knot in the rope (whether while rappelling or hauling). This will generally be more time consuming than just having an extra length of rope for which to affect the rescue and it should be noted that the knots that were tied to add friction on the lip of the crevasse during the fall will also add friction during the haul back out, perhaps becoming completely jammed up on the lip.
As this is a standard self-rescue technique, I'll give a brief overview of how this might look for a typical 3:1 z-drag setup. The additional gear requirements are minimal, needing only an extra sling/prusik and two carabiners.
Always obtain professional instruction
For variation on this approach, see the following technical bulletin from Petzl: Crevasse Hauling on a Rope with Knots. While illustrated with a Micro-traxion and a tibloc, the same procedure works with prusiks as well.
It proceeds much in the same manner for steps 1 and 2, but then uses an extra sling to connect the hauling prusik to the anchor (shown in green):
Pull on the hauling system to release tension on the braking mechanism (whether micro-traxion or prusik), and then allow the rope to slide back through the system until the weight of the climber is taken up by the green sling. With this newly introduced slack, untie the knot (or pass it by extending the pulley at the anchor), then haul again until the green sling is slack and can be removed.
