I have come across the following issue with two clients in the past two days. Previously (before Orion?) if you added a person to a list who was already a member of the list, Marketo did not "add them again", i.e. it would not trigger an Added to List event. Now it does. So if you have multiple triggers in a campaign, one of which is Added to List, and in the flow of your campaign you do an Add to List action to the same list (because it may have been triggered by one of the other trigger events, and you want to keep your lists consistent), the campaign will trigger again if it is not a "run through once only" or "run through once every <time period>".
Marketo then happily runs them through an infinite number of times, filling up their activity logs with repeated runs through the campaign.
To avoid this, either change the schedule to run every x hours/days/once only, or if this logic doesn't apply, you need to check for membership of the list as a choice in the flow before you add them to the list.
You can see if this is happening by checking your campaign queue - I noticed it with one customer because the segmentation wasn't being calculated in a timely manner and when I checked the queue, found that one triggered campaign didn't seem to be decreasing the number of leads that were running through it, and it was sitting at the top of the queue, holding everything else up (including the segmentation).
Hope that helps others avoid the pitfall! I didn't see any notice that this behaviour was changing, but maybe I missed it somewhere.