Does it safe to convert bytes to string with UTF8 encoding?

Today I saw a question with code like this:

var accumulator = "";
var buffer = new byte[8192];
while (true)
{
    var readed = stream.Read(buffer, 0, buffer.Length);
    accumulator += Encoding.UTF8.GetString(buffer, 0, readed);
    if (readed < buffer.Length)
        break;
}
var result = Encoding.UTF8.GetBytes(accumulator);

I know that this code is inefficient but does it safe? Is there some byte sequence that break the result?