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Is it

I might be the worst thing that would have ever happened to you

Or

I might be the worst thing that would have ever had happened to you

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These are your best choices:

I might be the worst thing that ever happens to you.
I might be the worst thing that ever happened to you.
I might have been the worst thing that ever happened to you.

fuchsia car 309I might be the worst thing that would have ever happened to you.

Possible, but awkward. I don't advise that you use this.

fuchsia car 309I might be the worst thing that would have ever had happened to you

Impossible.

would have had happened is ungrammatical.

CJ

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CalifJimfuchsia car 309I might be the worst thing that would have ever had happened to you Impossible.would have had happened is ungrammatical.CJ

Is I might have been the worst thing that had happened to you grammatical?

anonymousIs I might have been the worst thing that had happened to you grammatical?

Yes.

(But it sounds a bit weird to me.)

CJ