The bugs are way more subtle than that. You're lucky if your bad programming is caught somewhere at runtime. In most cases, Python and other weakly-typed languages will do everything they can to fit the data you're providing with what the code is expecting. This means converting strings to numbers and the other way around, plus a lot of other oddities you would not always expect (see PHP's intval for a crash course in bad language design). All that results in bugs that are far from obvious, and that don't always raise neat exceptions.

EDIT: I stand corrected, I don't know enough about Python in particular. The argument is still valid for other languages with weak types.