Synonym for rare occurrence but with negative connotation

Question

What are some synonyms for 'rare' or 'unusual' that have a negative connotation. For instance:

Enron's collapse was unusual, as the massive financial trickery ultimately bankrupted millions of investors.

What could replace 'unusual'?

Answer 1

I think Aberrant may be my top choice, perhaps merely on account of it containing 'err', in this context. Others that may be appropriate depending on the emotion related would be:

Strange

Unimaginable

Unthinkable

Answer 2

Abnormal
Adj: Deviating from what is normal or usual, typically in a way that is undesirable or worrying.

or perhaps:

Pernicious
Adj: Having a harmful effect, especially in a gradual or subtle way.

Answer 3

Catastrophic: While Enron's collapse was certainly bad, catastrophic might be a bit over-blown, depending on what point the writer is trying to make.

Devastating

Calamitous

Ruinous and disastrous may be slightly weaker than catastrophic and devastating.

While all these possibilities do not explicitly mean "rare", I think most people understand that disastrous events are rare by their very nature.