Mutable strings are indeed sort of "strange", especially given Lisp semantic for literals that is different for example from Python (in python the syntax "[1,2,3]" is a list literal, but is indeed equivalent to Lisp "(list 1 2 3)" and not to "'(1 2 3)", i.e. returns a new fresh list at each evaluation). Mutable literals is somewhat counterintuitive and in Python a similar problem happens with default values for parameters (i.e. "def foo(x=[1,2,3]):...") not because the literal is altered but because default value is evaluated only once at function definition time. This idea that what is in the program text is not in this case fixed is a trap in which newbies often fall...
