Is resonance always stabilizing 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?
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.