I've always known about this and taught it to quite a few people older than me, but I just looked up what temperature it happens from, and it's much lower than I expected. From a CRC Brakleen safety data sheet, it can happen as low as 150°C (302°F). The rest of Google reckons around 300°C...
Watch the temperature of those cables that have been bundled up. It probably won't be a problem due to the chargers not pulling that much power, but a cord bundled up like that can overheat.