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NG0900 is a generic error that occurs when attempting to compare objects of different types in Angular. In this case, it is likely that the code is trying to compare an object with the string "object Object," which is not possible.

Regarding the second question, observables and subscribe are part of the RxJS library, which is designed to work with collections of data. As such, it is optimized to work with iterable or array-like data structures. Other data types, such as objects or primitive values, may not be supported, which could result in errors like the NG0900.