A lot of cryptocurrencies is using Secp256k1 (for example: https://en.bitcoin.it/wiki/Secp256k1).

Every cryptocurrency library comes with its own redundant implementation of Secp256k1, ECDSA, RIPEMD160, SHA256. So there can be some inconsistency across implementation.

What if some library generates private key using different curve than Secp256k1. Eg. some with less security, or even a backdoor. Choosing different points, etc.

Then they share only public key with blockchain, of course. Is it possible that such thing would work? Can some public key appear valid for Secp256k1, but in fact use another elliptic curve?

I just came across many opensource cryptocurrency libraries, where they generate private keys and everyone is using custom implementations of these. So if I should at least compare source code and look for some specific set of constants.

Many cryptocurrencies use Secp256k1.

Every cryptocurrency library comes with its own redundant implementation of Secp256k1, ECDSA, RIPEMD160, and SHA256. So, there can be some inconsistencies across implementations.

What if some library generates a private key using a different curve than Secp256k1? For example, one with less security, or even a backdoor. Choosing different points, etc.

Then they share only the public key with blockchain, of course. Is it possible that such a thing would work? Can some public key appear valid for Secp256k1, but in fact use another elliptic curve?

I just came across many open source cryptocurrency libraries where they generate private keys and everyone is using custom implementations of these. So I should at least compare source code and look for some specific set of constants.