If you have a team that is big enough and a project that lives long enough that someone has to do changes in some old code, this thing is bound to happen sooner or later.
People have to distinguish between the simple issues that trip up beginners, and the fundamental issues that plague you for life.
Whether it causes any issues in practice for some particular programmer or not (and 10K lines is not much even for ONE modern webapp, and you write them broken down in multiple libraries), the thing is the behavior is logically flawed in itself.
Like Javascript needing "var" to NOT make a global variable (instead of defaulting on local vars) is.
Coffescript's behavior on this scoping issue is in the list of bad items that includes goto, global variables and nullity.