…leave it in the same condition you found it.
Right, it sounds like shit. But what is the grammar error called?
I don’t think it’s an error. It’s common and idiomatic in English to leave out the relative pronoun in a relative clause. The sentence could also be written, “leave it in the same condition that you found it”, but dropping “that” is fine.
It’s missing something – the error is it’s incomplete, and not in the normal way that certain words are assumed but unspoken. I don’t think a native English speaker would say it that way. It should be either:
A) Leave it in the same condition you found it in.
or
B) Leave it in the same condition in which you found it.
I side with Giles. Dropping words is extremely common in informal speech. It’s not an error and not bad English in any non-formal setting.
So it’s the missing preposition “in” that’s the problem. I missed that because leaving out a second preposition that duplicates an earlier preposition sounds right to me, as it does in:
Leave it on the same shelf that you found it. [Dropping “on” at the end.]
My other thought is that “as” can be used instead of “that”, and you could still drop the final preposition:
Leave it in the same condition as you found it.
Leave it on the same shelf as you found it.
However it is conceivable that these also sound wrong to other native speakers than me: English grammar is not an exact science.
I don’t agree. I don’t think the “leave it” and “found it” portions of the sentence are equivalent in that way.
To me, “leave it” is the primary portion of the sentence and “in the same condition you found it” is just a modifier to that. “Leave it in the same condition you found it” is the equivalent of “Leave it in the upright position” or “Leave it in the recycling bin next to the door”.
If you view “in the same condition you found it” as a modifying phrase, there’s no need for a second “in”. “In the same condition you found it in” would be like saying “next to the place it’s next to”.
If it ain’t broke don’t fix it. 
All of the suggestions sound OK but the original sentence OP disliked also seems fine to me and, although slightly informal, most idiomatic. 
This should be in IMHO, but here goes. First off, if you study formal linguistics you will find that the relativizer “that” is, in many contexts, optional whether in formal or informal speech/writing. As for the omission of the “in” I would usually put it in, but I see no problem dropping it either. The original sentence is perfectly acceptable.
I wouldn’t say it’s a grammatical error. Stylistically, it’s an “elision”.
But I found it in a condition. I’d have to go with “Leave it in the condition you found it in”.
It was in the condition you found it.
Now leave it that way.
Leave it in the condition you found it.
Leave it as you found it.
Nope, I found the thing, not its condition. “Leave it in…found it in” has a nice parallel.
It’s a false parallel. The sentence is about how you should leave it. It isn’t telling you how you should find it. Leave it is the main part of the sentence and in the same condition you found it is a prepositional phrase. The in at the beginning of the phrase connects it to the main part of the sentence that it is modifying. A second in would be redundant.
Here’s another example. Suppose you lost your key ring and then found it. You might then say “I found my key ring in the desk drawer.” But you wouldn’t say “I found my key ring in the desk drawer it was in.”
A parallel construction of this idea would be “If you find it in good condition, then you leave it in good condition.” Now you have an if/then which is creating an equal status between you find it and you leave it so now both halves justify having their own in good condition modifier.
I’m sure you’re right but I will have to stew on this and get used to it! 
“Leave it in the same condition in which you found it” would be most correct to me but I would find the original of “leave it in the same condition you found it” understandable and completely acceptable.
“Same” is redundant there, surely? “Leave it as you found it” or “leave it in the condition in which you found it” both seem better to me.
Leave it the way you found it.
But a prepositional phrase doesn’t modify in the same way as an adjective. There are two ins, one for each underlying verb collocation.
A) I found the bike in good shape.
B) I left the bike in good shape.
Technically, leaving out the *in *for A) is like saying: I found the bike good shape.
I think what’s happening here, though, which makes the omission acceptable to most people, is that the prepositional phrase in [condition] is being parsed like an adjective (a true modifer, as Little Nemo suggests), which doesn’t take a preposition.
A) I found the kitchen dirty.
B) I left the kitchen dirty.
I left the kitchen as I found it.
(…which is another way of saying what dougie_monty suggests above–Yes, it does the trick, but it doesn’t answer the OP.)
Part of this, I would say, is because the relative pronoun is dropped, but it’s also because I think that “stranding” prepositions at the end of a clause, (you know, the way you were always forbidden from doing), seems to be less preferred when there is an intervening pronoun (it). That’s just a hunch; I’d like to do a corpus search to see if it pans out.