The type of an underlying variable at runtime may be different than what the typescript type declares. Practically, this would probably be due to either poor, incorrect, out-of-sync, or incomplete type definitions.

A poor example, but this compiles without error and crashes at runtime:

let x: string = "foo"; (x as any) = 1; x.substr(0);

Now, technically, you can do this sort of thing in most, if not all, languages, but this happens organically more often with typescript due to many libraries being written for plain javascript and being looser on their types, or type definitions getting out of sync with the libraries they are written for.