If your camera uses a flash, make sure you have everyone's okay to use it. That includes the person standing behind your actual subject, who may not want a bright light flashed in her face.
If you aren't taking a picture of me, and give me some warning, I can turn away or close my eyes. It's not that difficult to ask permission, or to say "flash warning" if someone who has previously talked to you about this is present. And if there's a roomful of people, there's a nontrivial chance that one of them will be bothered by the flash.
And don't expect people to justify that request: "it bothers me" is sufficient. Nobody owes you their medical history in order to not have you take unwanted flash photos. This is less of an issue than it was ten years ago, thanks to current digital camera technology, but there are still people who assume that merely possessing a camera overrides anyone else's desires. I hope e-hellions know better: but if someone complains to you "would you believe that woman didn't want me to use their flash?!" it would be a kindness to explain that they shouldn't hurt random people in order to grab a few snapshots. Yes, for some people, bright light in the eyes can be painful and/or mean they can't see anything for a couple of minutes.