There are none, right? So even if you add mutexes to the existing code in an attempt to remove race conditions, it will still have race conditions, and have no guaranteed output order, because you have no guarantee which order the goroutines will start in.

The obvious way to solve it in order to get the answer shown in the source code is

    if x == 0 {
        *sequence = []Button{{"red"}, {"blue"}, {"green"}, {"yellow"}, {"purple"}}
    }
    return

This way there is nothing concurrently touching sequence, nor is there anything concurrent touching rand, nor is anything relying on goroutine starting order, nor are there other race conditions. But yet the site says this is wrong.