I noticed this bug while using the shadowdancer. And I can't even reproduce it perfectly, but here's how it usually happens:
1) I use Shadowstep 2) During Shadowstep, I activate stealth mode. 3) Just when it's about to end or just after it ends, I order the character to attack whatever I want to backstab.
Result: You become visible while doing the attack animation, but without actually hitting the enemy (or missing the enemy), ruining the opportunity for the backstab. Using an invisibility potion during shadowstep also can make this happen. I don't think I ever saw this happen in any other circumstance. It doesn't happen very often, I suspect it's all about the timing, but it's annoying nonetheless.