Using proper technique is important when putting vias in pads. It's also important when putting vias near pads. With BGAs, you pretty much always have to put vias either in or near the land pads. Obviously, we prefer near rather then in, but care must still be taken. First, the geometries are annoyingly small - especially if you have one of those .5mm pitch BGAs. Second, if your soldermask doesn't break the path from the pad to the via, you can have the same problem as a via in pad setting; the via can wick the solder away from the pad and down the via.
To prevent this from happening, make sure that the area around the land pad is completely masked. If the via touches the land pad, make sure that the solder mask either covers the entire via or at least the annular ring.
In these two board top-views, you can see actual boards illustrating the incorrect method (B) and correct method (A) for placing vias near BGA pads. I didn't have an example of the incorrect method lying around to photograph, so image "B" is a simulated case. But that's pretty much what it would look like.
Sometimes you can get away with this when using lead-free solder, because it doesn't flow as much, but it's best to stick with the correct method, as in example "A".