I've found it impossible to catch a Romulan in a lie...
I've managed to catch numerous Romulan "fabrications of a truth", and equally numerous Romulan "interpretations" of the truth, but never a flat-out Romulan lie...
Then again, the Romulan statement "Romulans do not lie" could either be a lie, or yet another misleading fact, clearly illustrating that they never lie, they just make up their own truth as they go along...