Are both santences natural?

Don't say you didn't warn me when something undesirable happens.
Don't say you didn't warn me when an undesirable situation occurs.

Question

Are both santences natural?

Don't say you didn't warn me when something undesirable happens.
Don't say you didn't warn me when an undesirable situation occurs.

Answer

No, there are problems here. I think you mean to say:
"Don't say I didn't warn you..."

Next, bear in mind that "undesirable" is a formal word, while "don't say I didn't warn you" is an informal phrase. They don't belong in the same sentence together.

So, to make the whole sentence natural in casual speech:
"Don't say I didn't warn you, if something bad happens."

Não perca a oportunidade de aprender uma língua no conforto da sua casa. Navegue pela nossa seleção de professores de línguas experientes e inscreva-se na sua primeira aula agora mesmo!