Forex.com: Ask < Bid

I've been noticing that at certain instances the ask price is less than the bid price. What does this mean and is it a regular occurrence? Or is it a glitch in the quotes that I am receiving?

 

Don't use them, they are a bucketshop.

 

Its called "shading" the market.

 

I have never heard this term. Can you explain how "shading" works?

 

So are you saying that this is not a technical glitch?

I looked up the term "shading" the practice of brokers manipulating prices seems to be common. (http://www.investopedia.com/articles/forex/09/price-shading.asp) However, it makes no specific mention brokers creating a negative spread. Wouldn't they lose money by doing so??

Is it possible to actually purchase a currency when the ask price dips below the bid price?