What's wrong with a little grumbling or complaining?

Don't be grumbler.

What's wrong with a little grumbling or complaining?
And when the people complained, it displeased the LORD: and the LORD heard it; and his anger was kindled; and the fire of the LORD burnt among them, and consumed them that were in the uttermost parts of the camp.
Numbers 11:1 
Doesn't sound like it pleases the Lord...
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Nor let us try the Lord, as some of them did,
and were destroyed by the serpents.
Nor grumble, as some of them did, and were
destroyed by the destroyer.
Now these things happened to them as an example,
and they were written for our instruction, upon
whom the ends of the ages have come.
1 Corinthians 10:9-11 
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The Lord will judge grumbling... let's learn by example
Don't grumble against each other, brothers, or you will be judged. The Judge is standing at the door!
James 5:9 
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Thanks be unto God for his amazing gift:
Jesus Christ, the only begotten
Son of God is the object of our faith;
the only faith that saves is faith in Him.