I'm up early so my holiday present to all of you is the gift of knowledge. Sure it's cheap, but I made it myself.
I have met a few magicians in my life who actually don't care if they flash while performing. They do nothing to fix it nor do they change anything. At that point, they are just there for themselves and the self-gratification it gives them. Let me tell you why it's bad to flash people.
Not only does it show a lack of skill and proper practice on your part in their mind, but it pulls the audience right out of the moment. It's kinda like watching House of 1000 Corpses. Yer into, yer into it, WHAM!! music video sequence.... wtf?
When someone watches you they are focusing on you. They are (hopefully) with you all the way in your little world for those precious few minutes. However, the moment they get a glimpse of something they shouldn't their focus is broken and they are pulled right out of the moment and come hurtling back to reality. This is bad.
This is why having a manipulation act can be a treacherous mistress. If you mess up for magicians, we are more forgiving (sometimes) than a non-magi audience might be. We already know it's BS and skill. They don't. The same holds true for you close-up guys too. Those of you who rely too much on sleight of hand to accomplish your feats because you are so very proud of them and "Dammit I practiced this for 3 months straight to get it down most of the time so I am gonna do it and no one is gonna stop me". The problem is the same. You screw up and flash something, you are not only hurting yourself and the people watching, you are boning ALL of magic (to a degree).
So consider this when structuring your routines both on stage and off. You'll know if you're doing it right because you won't see anyone leaning over to whisper in their friend/spouse's ear and point at your "empty" left hand.
It's like grabbing someone by the shirt in the middle of a card trick and then licking their nose suddenly...
no one wants that do they?