Beginning with a flashback or dream
I am guilty of this, one of my first beginnings (yes, there was a previous and I’m on iteration seven) started with a FATS (Firearm Training Scenarios) scenario. Only, I didn’t reveal it wasn’t “real” (fictional reality) in the story. The jig was up when the proctor called an end to the program.The jig was up for me when I got called on starting with a dream.
James N Frey, author of How to Write a Damn Good Novel , really cautions against flashbacks. He does admit it’s not absolute. His article On Flashbacks is worth reading. The skill comes in “bleeding” in the backstory at the right time in the right way.
For more on handling Flashbacks see Flogging the Quill.
Nullius in Verba 
What about ‘Last night I dreamt I went to Manderlay again’? (Rebecca).
Does that count?
I don’t think so. I think this is talking about the opening sequence where, for instance, the person flies over the tops of the trees, robs a bank, makes love to someone, and then wakes in the bed to find it was all a dream.
‘Last night I dreamt I went to Manderlay again’ is the character’s state of mind and thoughts. That’s dialog.
Is your cold better?
And have you always had word verification?
My head cold is going away slowly. The throat is almost back to normal.
I started the word verification when one of my posts picked up two spammer bot posts in two days. I hated to do it. WV is so annoying.
What about a flashforward? Is that a yea or a nay to open a book? (she asks, having just written an opening with one).
I’m not sure. My story had a “present time” action but needed too much information preceding it that it became a flashback.
Do you mean that?
Or perhaps you have a prologue and then flash forward to present day? Prologues are not the top ten list but the people who don’t like them are legion.
We know there are exceptions to all writing “rules” but knowing the risks of trying such things helps.