XPages: Obscure error message for undeclared variable

I've had an error message that popped up regularly in our logs.

com.ibm.jscript.InterpretException: Script interpreter error, line=524, col=25: Interpret exception
 at com.ibm.xsp.javascript.AbstractLocalObject.put(AbstractLocalObject.java:95)

The line it refers to has a for .. in loop on a scoped variable. I believed it had to do with the object being in a scoped variable, and tried to make a by-value copy. That didn't help.

I finally (!!) discovered the reason for the error message. I had forgotten to declare the "key" variable.

// Results in error
for( key in object ){ .. }
// OK (key can also be declared outside loop)
for( var key in object ){ .. }