A render texture that renders itself causes a crash when it is set to 15000x15000

Render texture of 15000x15000 pixels in size will most likely consume so much graphics memory that it'll simply run out of vidmem. Just the color buffer alone takes some 850megs, plus depth and whatever other memory structures the GPU will need for that. No wonder it'll crash =). Unfortunately we cannot reliably detect all of these cases, so there's always going to be situations where Unity will just crash when it runs out of vidmem.