I have the Lexa 300 CC (Blue) in left hand, it's similar to the 300 HD (blue) reel, or I guess also the orignal Lexa 300/400. I've had it a few months and finally used it last weekend and today. 

 

This is the problem and I don't know if it's just the reel I have or it's the whole series. I press the thumbar, no problems casting. Then out of nowhere, I make a cast and the thumbar(I guess this is the clutch?) engages and sounds like the gears are going to get jacked up. I thought maybe I didn't press the thumbar good, and the times I did; it would every now and then engage and stop the cast and scare the poop outta me cuz I think the reel just imploded.

 

Have you experienced this with your Lexa 300 or 400? I have the the paddle handle model, not the power handle. 

 

 

Exchange it for another.  

That shouldn't be happening with a paddle handle.

 

Anyhow, if you keep your thumb firmly on the thumb bar during the cast, it won't happen again. It's a good habit to get into, in case you actually do end up using a power handle for anything.

Exchange it under warranty.  Definitely not normal.

Yep. Same for me. I did some Google searches a while back and other people besides us have also had that same problem. I lost an 8" Hud and my favorite glide bait due to that issue, and now I've been doing just as portiabrat mentioned without any further issues (yet). I'm planning on upgrading to a Tranx during the BF sales so I can relax a little more while casting hundreds of times in hopes of a single giant bite.

Mine had that same problem. 

It's a known issue with the larger Lexas. Mine has the same issue. Generally I cast with the knuckle of my thumb on the thumb bar anyway so I can thumb the spool, and make a little extra effort to keep the thumb bar pressed down all the way through the cast. So it's not really an issue for me unless I get lazy and  take my thumb off in which case it has happened to me a small handful of times. If it is under warranty exchange it. I've heard of guys having the same issue after exchanging it multiple times, and others that never encountered it again. Try your luck. 

Known issue with Lexa’s in general. I had two 100’s a few years ago that would do it now and then.