The X/Y Problem is when someone asks for help with their solution rather than asking for help with their problem. That is, they are trying to solve problem X and they think that solution Y would work but they're having trouble implementing Y. So when they run into trouble, they ask for help with Y when really what they need help with is X.
This often causes frustration because you won't understand why they're trying to do whatever it is they're doing. Imagine that someone is trying to get something to print and they're convinced that it is a paper jam causing the problem. They've looked in all the normal places and now they're asking for help disassembling their printer to find a piece of paper they can't seem to find. Finally, after hours of trying to help, you ask the fateful question, "Is the printer plugged in?"
This problem is particularly challenging from the point-of-view of the support staff because some people will see your attempts to change the flow of the investigation as being unwilling to help. Sometimes you will have to play along until the person asking for help reaches a point of frustration or some other teachable moment. When that happens, you can ask to start over and find out what really started them on this journey. I've found the best way to do this is to ask people what they're ultimately trying to get done, not how they're trying to make it happen. Then you can guide them in the correct direction.
See also: Einstellung Effect