When we accept Christ into our lives and become born again, that does not mean He has waved some sort of magic wand and made us sin free. Think of it like this...you have a nice, clean, crisp, beautiful white shirt...it looks absolutely gorgeous the first time you put it on, you're so proud of it and you want everyone to see it. They think it's beautiful too...but you wear this shirt everyday, you don't wash it, you don't take care of it...it gets rained on...you spill food on it...you fall into a mud hole while wearing it...it's soiled, it stinks and looks like it should be thrown away, like there is no hope for this shirt...then one day you throw it in the wash, with some bleach, and it comes out looking brand new. And you are able to wear it again, and it looks beautiful again. But, just because it's now clean, that doesn't mean it can't get stained again...so at lunch you have a hot dog and you spill mustard on your pretty white shirt.
By your logic once the shirt is washed it can never be stained again. So, tell me...when the mustard spilled on your pretty white shirt did it just keep on rolling down to the floor? Or did it set in and stain your shirt so it had to be washed again?
The only way it would be possible for the shirt to not become stained again would be after you washed it to cover it in plastic and hang it in your closet...but, you know...even then something catastrophic could happen, like maybe a storm comes through and a tree falls on the house, right over the closet. The tree rips through the plastic, exposing the shirt...now it's covered in debris, and tattered and torn. So, in all actuality, there is always a chance that this shirt will get dirty again, no matter how much you try to protect it.
Now, this is a very simplified analogy, but the point is this...no matter how faithful one is, no matter how righteous, they are human. And, as a human they have a sinful nature, and at times are going to mess up. It could be as simple as rolling your eyes at your wife, or calling someone an idiot when they cut you off in traffic, but we all fall short of the glory of God. But, as Believers, when we fall, we recognize that we have and we repent of our sin, we seek counsel from the Holy Spirit to keep us from making that error again.
I would say that anyone who claims to be blameless needs to search themselves and ask God to shed light on the area of their life that needs to be improved on...pride and boastfulness are sins unto themselves...