R5RS defines many things as "is an error" without any specification of what happens in that situation. R6RS goes to the opposite extreme and specifies as much as possible what exceptions are raised when.
Taking into account the system provided by ticket #18, we need to come up with guidelines for when exceptions should be raised, and clarify which R5RS "error" situations should raise an exception or be left unspecified.
WG1 voted for the R5RS status quo.
"Is an error" is fine for WG1. I see WG1 Scheme as vary much a "seat belts off" language. One's program shouldn't be doing things which create "is an error" situations anyway.