>jrunscript
nashorn> bbb=1
1
nashorn> bbb==1 (1)<-------------
true
nashorn> delete bbb
true
nashorn> bbb==1
false
nashorn>

>jrunscript
nashorn> bbb=1
1
nashorn> delete bbb
true
nashorn> bbb==1
script error: ReferenceError: "bbb" is not defined in <STDIN> at line number 1
nashorn>

They differ only in the execution of the statement 1.
This is only a test, and as such it should not tamper the symbol table of the engine. However, it seems that it does because it makes the last statement executed behave differently: in the first case, the symbol bbb seems to resist to its deletion, while in the second case it is actually deleted (and a reference to it rejected with an exception).