Has anybody done this by accident, and were there any negative effects?

you can not install a diff gear on a Losi backwards it comes down to what side you are happy with the adjustment screw on. Losi recommends putting it on so the adjustment screw is on the Left side for easy adjustment.

Not true.. If you put it on the right, the diff will unscrew itself in only a few minutes. It MUST go on the lefthand side, or else it won't work.

Here's what's happening to me:
The front is installed w/ screw on the left side, I'm told this is the correct way. The rear WAS installed in the opposite fasion unknowingly. While racing with this setup I was being lapped like I was standing still. I just noticed this improper installation last night and switched it. I havn't had a chance to race it yet and cannot tell is the speed is back when I run it alone in the street, I need to be side-by-side with other cars. SO, I wont get to see the results untill the next time I race. I was just wondering what some of your opinions might be. Thanks.

Was it barking? If the diff screw was loosening up, then it would bark when you land or take off. And how was the gear itself? If it was loose, then the diff balls would have heated up and melted themselves into the gear.

Btw, the diff I'm talking about is for a XXX-T.. The XXX-4 may be able to go either way, I've never touched on of those. I do know the XXX-T diff MUST be installed with the screw on the left side.

the info I posted was out of the XXX-4 manual so the info I posted was true according to the Losi manual, I have ran Mine on the right side for 4 months and never had a problem.

I'm talking about my XXX-S, the manual says to have the screw side to the left. There has been no barking and no melting, but I've kept the diffs pretty tight. I've got tomorrow off and plan to play with it a little. I'm hoping the backwards diff was the cause of the slow top speed but I'm not going to hold my breath. I'll let you all know how it goes. Thanks for the advice.

Eworx, well when I first got my XXX-T, I put mine in backwards.. Before the first pack was dead, the diff was barking. I assumed it was just breaking in, so I retightened it and ran another pack.. Same thing.. Man, this thing takes a long time to break in, right? Retightened it.. I must have retightened the thing 10 times before I finally gave up and flipped it over. It worked fine once I did that. You must have some SPECIAL diff that works either way, cuz the rest of us got one that needs to be on the left..

WOW....

After a month of slow driving, my LHS spent two hours looking over my car...it was the ESC! A GT7, programmed at minimal throttle. After resetting it to full throttle We then discovered that it had no brakes. After all that, I ended up having to buy a new one. $140 later, I now have full throttle AND brakes. It wasn't the diffs, bearings, or batteries. I wish now that I had checked the ESC way earlier. But at least I'm back in the game.

They couldn't replace the esc for free under warranty?