I got 34 units of Botox in my forehead and for ptosis in one eye. The doctor said she did everything right.

I would say that it was more likely related to injection technique and possibly post-injection behavior. I don't think this means you cannot get Botox in the future with excellent results.

Although the location of the injections and the dosage seems okay, The size of your forehead and the strength of your forehead muscles matter how much you inject. Diffusion of the neurotoxin can be an issue

Ptosis after botox injection in the forehead is a well know though rare complication of injection in of neuromodulators in the forehead. In the picture you sent it seems your Doctor did everything right but it is possible that your Botox migrated or your anatomy is little different. I would not hesitate to get Botox again but knowing that you did get ptosis your doctor will inject a little differentially. Fortunately ptosis does not last too long. I recommend the people that I inject to not to stoop down or put their head down or lie supine for 4 hours after the injection. Best of Luck to you next time.

Unfortunately ptosis after botox is an annoying complication. In the clinical trials of botox and similar products, they actually allow up to 2% risk of this occurring as acceptable. Thankfully, it's usually just a couple weeks that patients typically have this issue before the muscle recovers. 
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The only way to be 100% sure ptosis will never happen again is to never get botox ever gain. Just because ptosis occurs once does NOT put you at any higher risk of this happening again.Statistically, this is very unlikely to happen again. Ptosis can occur even in the best of hands and obviously in less than the best, it will occur more often. 

So most any injector who has any experience will have caused ptosis. It is rare for a skilled injector though.... I would not be scared from doing botox again just because of ptosis- if it happens twice then I suppose it is possible that your anatomy is varied but it is typically the injection technique 

Ieyelid ptosis does happen even though extremely rarely. It's quite possible that when you come back to the same doctor, knowing what happened to you before, she will change her techniques slightly to avoid this complication. Good luck from New York City

There is always a chance of brow ptosis with injections- but in the end it is dependent on where you actually receive the injections. From the illustration that you provided, it appears that you were injected in the correct locations in order to avoid ptosis, however you may have received too much in your forehead or the injections that you received between your eye brows could have accidentally been injected into the muscles that is around your eye- everyone's muscles are different but mistakes can easily be avoided with proper assessment prior to injecting. My recommendation for you is to have them inject a little less in the forehead and extra attention be made when injecting between your brows. 
Best of luck. 

Getting a ptosis is a risk, although very small, of getting botox. I think you should be safe to try botox again, but I would decrease your number of units. Somehow the botox migrated to the muscle of your eyelid and affected it. Also make sure to avoid any heavy exercise, inversions, and laying on the face for at least 4 hours after injections.