I had my bow propped up against a wall with an arrow in it while getting situated in the blind. The bow fell over and snapped the arrow in half. I found this surprising as i thought carbon arrows would have been tougher.

It's a Cabelas bow hunter, which is made by Easton. 340g.

I have had them break but not that easily - may have had a defect

I had one hit a ~1/8" twig on the way to a turkey last year.  





Missed the turkey since the arrow broke cleanly in half and fell to the ground about 10 feet in front of the turkey.  I've never seen or heard of it happening to anyone else until now.
That was an Easton Epic? or something carbon, must've been a factory defect that finally showed up since I'd shot & inspected it several times prior.





ETA:



Woops, necro thread!

To answer the original question, yes, I have. However, it has only happened when the tips were soaked in acetone to remove the inserts. This only resulted in a clean break near the insert, not mid-shaft.

I shot a buck Saturday straight down as it was under my stand.  Arrow clipped his spine.  I found the back half and front half off the arrow laying on the ground (clean across break).  I thought the arrow might have passed through and stuck in the ground, then the deer broke the arrow when he landed on it.

However, when I was processing the deer, I found two more sections of arrow inside the deer, sort of lodged in the spine, each piece about an inch long.  Broken clean across at each of the ends.  Now I'm thinking the arrow broke at the spine, the front half passed through and hit the ground, then the back half broke off when he rolled upside down hitting the ground, leaving the two smaller pieces of the arrow shaft still in the deer.

I had one snap clean on let off about 3" from the wrap. Heard a pow, knew my bow had effectively dry-fired, heard a weird whizzing away from the bow, and felt a lot of pain in the top of my hand between the first and second knuckle. I almost didn't want to look at my hand due to all the horror stories and pics around the net. Lucky for me I just had a lot of bruising and a slight skin break. Never did find the front of the arrow. FWIW I always flex my arrows...everytime. This just made me realize it is a good idea to retire arrows from time to time.