A query letter is a pitch to an agent to read your book.
It's not a deposition, or even a frank conversation with your dear mum.
In other words, you don't have to tell the whole complete, every last detail of truth.
You don't need to say it took you six years to write this frigging novel.
You really don't want to say that you never want to look at it again.
You can gloss over previous publishing efforts if they don't have a public face.
You should leave out that you think agents are blood-sucking leeches and clearly have limited taste since your book hasn't been snatched up with glee yet.
Lying is telling me you've never published anything if you have.
Lying is telling me you've never had an agent if you have.
Lying is telling me you have an offer when you don't. (I've opined on that before)
Lying is never a good idea.
And then there's just plain ignorance, and that's the one that can really trip you up.
Ignorance is telling me you won an award that doesn't exist.
Ignorance is telling me you trademarked your book (when you mean copyright, and don't do that before querying anyway.)
Ignorance is telling me an agent liked your work but was too busy to take it on (you didn't recognize a form rejection.)
Lying is an automatic pass, but so is ignorance.
I'm very reluctant to take on people who don't know what they don't know.
It's one of the reasons I'm very glad to answer questions (if I can) here on the blog.