Arc "feels" a lot more Scheme-like than Common Lisp-like, but pg is a CL guy afaik; my observations on the two:
Scheme avoids type errors by writing programs which could be given static types with a sufficiently powerful type-checker. If a Scheme programmer needs to define two aggregates, both of which have a property "name," they're likely to define two different functions, foo-name and bar-name, to get at them. This, though it's noisy, makes type errors more obvious.
CLOS helps avoid type errors too, since it encourages thinking not about how code interacts with a single type, but instead how it interacts with a whole _protocol_ of methods. I think multimethods in general are a powerful design tool that help avoid type errors in a dynamic setting, but I haven't had a chance to try them out in a language other than CL.
Many CL implementations also have static type-checking. For example, when I try to define a function with a type error in SBCL:
* (defun f (x) (+ 1 x (car x)))
; in: DEFUN F
; (CAR X)
;
; caught WARNING:
; Derived type of X is
; (VALUES NUMBER &OPTIONAL),
; conflicting with its asserted type
; LIST.
; See also:
; The SBCL Manual, Node "Handling of Types"
;
; compilation unit finished
; caught 1 WARNING condition
CL's philosophy here differs a bit from most typed languages: SBCL will emit a warning (not an error!) for code it can statically show is impossible to run without getting a type error, while e.g. GHC gives an error for any code it can't statically show doesn't get a type error. However, in my experience, many type errors are obvious enough that SBCL warns about them (e.g. passing a vector where a list was expected, a string where a symbol was expected, nil where a non-nil value was expected, etc.), so this helps quite a bit, especially when combined with the interactive editing one gets through SLIME/slimv.
One can also attach type annotations to functions [0], and SBCL (and probably other implementations) will add a runtime CHECK-TYPE [1] unless you tell it to optimize for speed enough.
[0]: http://www.lispworks.com/documentation/HyperSpec/Body/d_ftyp...
[1]: http://www.lispworks.com/documentation/HyperSpec/Body/m_chec...