RNA interference and the former Petunias experiment >
They introduced a foreign gene responsible for petunia leaf colour, with the intentions of up-regulating gene expression for this colour, which was purple. What was observed however was intriguing. Rather than an up-regulation of the purple colour, the leaves were all white instead. This finding showed that the RNAi system in plants recognised the foreign gene and degraded it. Not only that, but the RNAi system also identified the endogenous equivalent of the foreign gene and down-regulated this also thinking it was foreign. This is how they discovered this miraculous finding :)