Got Temporary Veneers to Replace Bonding, and they Fell Out; is this Normal/Common?

You have nailed one the dilemma's of cosmetic dentistry squarely on it's head - how do we dentists effectively temporize teeth that have been prepared for thin porcelain veneers?  There is no good answer for you the patient or us the dentist.  What you need is a secure fit so they do not fall out in your daily life and what we need is to be able to remove those provisional teeth easily when you come into our office.  So yes your dentist is right...temporary veneers come off easily and is quite a common event (unfortunately :0 .)  This has no baring on your finally result.  You and your dentist do need to be careful about your bite in both cases (temporaries and final.)  Make sure your porcelain veneers feel very comfortable as you move your lower jaw back and forth in your normal biting patterns. You sound like you have a nice dentist, trust him and trust the process.  And if you have to go into the office again to re-cement your provisionals, make sure you do the bite test with the temps too while your dentist is there.  Best to you!   

We don't bond temporary veneers to the teeth, because we want to remove them easily when we’re ready to bond your permanent set. However, because of this fact, they can come off on you, especially if they’re made conservatively. This doesn’t mean that your permanent veneers will come off as well. Permanent veneers are bonded carefully to your teeth and they’re supposed to stay there for a long time. Be patient with them and work with your dentist.

Temporary veneers can sometimes come off sooner than your dental appointment to place your permanent veneers. Temps are placed and cemented to be able for the dentist to remove them without too much difficulty. However, this may also be a sign that your occlusion (bite) may need to be adjusted or other modifications to the size and length. Generally, the temporary veneers are shaped and used as a guide for the design and finish of your permanent veneers so if there is an issue with the temps, your dentist should evaluate their function and make any necessary modifications as well PRIOR to the placement of the permanent veneers. 

Of course, we HOPE they come off when we want them too and not earlier, but they CAN come off at home.  Our goal is to be conservative with veneers, but conservative veneers are not very retentive and require our bonding methods.  We don't WANT to bond in the temporaries...  then they WON'T come off.

The temporaries are more likely to stay put if we get more aggressive with removal of tooth structure, but then they become more like crowns.

Trust your dentist.  The finals are VERY likely to stay put.