I think Scala has a few tools that, like multiple inheritance in C++ or Haskell's MultiParameterTypeClass, need to have a bit of a warning label so people avoid them until the situation screams for it.
That said, yeah, UndecidableInstances look like a much better example. Especially when googling it turns up things like this: http://lukepalmer.wordpress.com/2008/04/08/stop-using-undeci...
That's where the similarity ends, though. CLOS does this at runtime during method invocation, MPTC "dispatch" is determined at compile time. Multiple dispatch will generally have some overhead that MPTC won't, but it's more flexible in that the set of function implementations can be extended without recompiling.
Also, due to limitations of type analysis or type expressivity, it's possible to write something that would work for the specific instances where you are using MPTC but can't be proven to work for all instances of the types involved. Then again this is almost always an issue with static typing. Wether the compiler is saving you from a bad decision or keeping you from doing your job is a matter of personal opinion.
You might be intressted in this paper I read just a couple of days ago: "Extending Dylan’s type system for better type inference and error detection". (Dylan OO is simular to CLOS) A system like that helps to get the dispatch overhead down while still beeing able to extend it at runtime and you get alot of the typesafty.