A couple of years ago James Goldie (UNSW) contacted me about an issue he found in HadISD relating to the reporting resolution of temperature and humidity information for stations in Australia.
In the HadISD, the data vary between single-degree, half-degree and 1/10th degree resolution. However, variations between these can cause some interesting striations in derived quantities.
James has written up his work, with some cool animated plots at his blog
