Does resonance always stabilize a molecule or can resonance effects destabilize molecules?

The only example I can think of in which resonance (more accurately, conjugation) destabilizes molecules is in the cases of anti-aromatic compounds. Is this a valid example, and are there any more such examples?

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    $\begingroup$ If resonance is not stabilizing, then why would it occur? $\endgroup$ – LDC3 Nov 1 '14 at 18:43
  • $\begingroup$ Couldn't you say that about a lot of things? If anti-aromatic compounds were unstable, why do they exist? I think the answer to your question lies with meta-stability. $\endgroup$ – Dissenter Nov 1 '14 at 18:53
  • $\begingroup$ What I was trying to point out is stated by Ron, Resonance only occurs when the result is beneficial. Anti-aromatic compounds exist since the molecular bonding releases energy from the state of individual atoms. $\endgroup$ – LDC3 Nov 1 '14 at 19:06

Resonance only occurs when the result is beneficial, that is when a more-stabilized molecule will result. If resonance (delocalization) occurred in a compound such as cyclobutadiene to produce a square molecule with equivalent bond lengths, an antiaromatic molecule would be the result. Therefore, resonance does not occur and a square molecule with equivalent bond lengths is not produced.

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