They come when you are least expecting it, waving their magic wands of "OhWeCouldDoThisToo" which are powerful though lacking in actual details of how to accomplish the ideas, what the costs may be or who should be actually working on such a task. Regularly, they use them as finishing touches on already well-thought out and planned events only days (or moments) from occurring.
You know who I mean, those bright eyed little sprites called 'Good Idea Fairies'. They come in, as always, both positive and negative variations. The positive variations are always trying to help. They are all smiles and regularly win awards for work they never actually have to implement. Most commonly, they are attracted to hardworkers who are placed just low enough on the authority scale to be unable to say their most hated word, "No" but high enough to do the work without needing a lot of management approvals. Beware their seeming understanding of the core topic, because inevitably they only know enough to be dangerous.
The negative sprites are more bold and easily spotted. They are in every meeting, attracted to the idea that something might actually be accomplished. Their job is to throw turds on the tables at every opportunity. Every reason why something might fail or not be accomplished is known to them, for they are well-read on the topic and understand the tenuous nuances of each layer. They are usually too busy (doing what, no one knows) to head any project and rarely have the answers to the questions they ask, but they are self-righteous in their adamant belief that if these issues are not addressed, everything is doomed. Anecdotal evidence of other similar failed enterprises are their Piece De Resistance in any debated situations. These sprites are sneaky in that after their initial appearance, they tend to disappear for a while until the previous issues are solved by which point, they have ferreted out more.
While their name sounds like something you would want in your life, Good Idea Fairies are like Fool's Gold. You think you have something valuable until realize it's just a shiny rock no one else wants. Most commonly found around upper management or costly endeavours, these little beings can upset any successful venture if not banished with logic, foresight, and common sense.