It was discovered by the information from a citizen. Any penalty? Nah. The Tokyo Metropolitan government told the wholesaler to try their best not to do that again.
From ANN News (4/27/2012):
It was revealed that a intermediate wholesaler of vegetables [who buys from a wholesaler and sells to a retailer] in Adachi-ku in Tokyo had sold cucumbers made in Fukushima Prefecture by putting "Made in Yamagata" stickers.
In September last year, intermediate wholesaler of vegetables "Daikane Bunki" in Adachi-ku, Tokyo sold 358 boxes of cucumbers made in Fukushima to retailers to be sold to the consumers. The company, when they packaged the cucumbers in small bags, put the "made in Yamagata" and "made in Iwate" stickers. The Tokyo metropolitan government investigated on the tip from a citizen. Daikane Bunki says, "We couldn't secure enough cucumbers from Yamagata and Iwate". So they packaged the cucumbers from Fukushima that they had in the inventory, and sold them. The Tokyo metropolitan government instructed the company that it should try not to do that again.
The wholesaler Daikane Bunki either took down their website or the site is inundated by irate consumers. Either way, it seems to have been removed.
Fukushima's cucumbers have been found with radioactive iodine and cesium, but the maximum amounts found (36 becquerels/kg of iodine-131, 12 becquerels/kg of cesium-134, 15 becquerels/kg of cesium-137) was from house cucumbers grown in Nihonmatsu City on March 24, 2011. Since then, they are mostly ND (Not Detected). (You can search the database set up by the Fukushima prefectural government, here (in Japanese only).)
Then again, the monitoring was a sample testing, picking one plot of farmland per village/town/city and pick one vegetable, washing it carefully and measuring it. Then the whole village/town/city was declared "safe", and the shipment of the vegetable was freely allowed when the vegetable tested below the provisional safety limits of 2,000 becquerels/kg of iodine-131, and 500 becquerels/kg of radioactive cesium.
In retrospect, it shouldn't have engendered much confidence among the consumers, but it did; back then not many consumers were aware of the sample testing method. I remember reading a tweet by a Fukushima farmer very early on in the crisis (in early April, maybe), warning against this testing method, saying 99.9% of the vegetables or fruits from Fukushima were not tested. I remember retweeting his tweet, but that fell on deaf years back then.
There are still people out there in the positions of power and authority - politicians of major cities (like the mayor of Yokohama who kept feeding her city's school children with cesium beef), or a kindergarten principal (who ended up feeding her kindergarteners with mushrooms loaded with radioactive cesium recently) - who continue to believe "What's being sold in the market is safe".