Depends which resource is being exceeded. Certainly if they try to exceed their RAM or CPU limits then the process will just die because there is no other action which could be taken and that will result in a 500, but what gets to the browser depends on what they've already sent by that stage. Exceeding other limits mostly just results in that action failing (eg. if they try to open a file past that limit then the file will not open so their code should test for that (as it always should anyway)).