> bKGD overflow, tRNS overflow

This is one of those awkward parts of the standard, the samples always take up 16 bits and decoders are expected to mask off the extra high bits, any 16-bit value was supposed to be considered valid. In the real world trash values are often written for transparency (tRNS) chunks that inadvertently match samples in the image when the extra bits are masked off, which leads to random holes in the image. In practice those chunks have to be ignored when the values are out-of-range for the given bit depth.