Inconsistent solder mask on vias
Could be sloppy design, could be the solder mask just didn't stick.

However, it is generally better practice to relieve the solder mask around vias especially if the board is to be flow soldered. Solder filling the via improves the quality of the connection through the board.

If you WANT the mask to cover the holes, you are asking a bit much of a fluid to consistently bridge over air. Add in heating processes during soldering and the chances of a few popping is pretty high.