Unexpected issues occasionally arise in meetings, for example, you might call a meeting to resolve a particular problem that has arisen rather suddenly. In other cases, it arises during discussion of something else, but everyone present realizes the need to make a decision.
In these cases, you usually have no specific motion to discuss, no specific information on which to base a decision, no proposal, and a relatively short time-span to make a decision. However, everyone in the meeting has an opinion. The amount of emotion and opinionization on the matter usually generates lots of discussion but hinders good decision-making.
First, prevention is better than cure. Some unexpected issues are unavoidable, but if they are quite common, you probably need to improve your planning and preparation for meetings. They are more typical in styles of unstable governance in which boards lurch from crisis to crisis.
Your worst possible outcome is probably to commit to an unsatisfactory decision that has long-term consequences. This is easy to do if people give a knee-jerk emotional response with inadequate information. Besides, people naturally tend to opt for the safe answer if asked to make a decision suddenly. They are cautious of making a decision that is creative or requires initiative, even if it is the best option.
If you are calling a meeting to address the matter, at least you have an opportunity to get somebody to gather some information and inform all meeting invitees on the issue.
Do you really need to make a decision now? This is one of the few situations where deferring the main decision is probably a good option.
If you have to make a decision now, can you make an interim decision and leave the main decision until you have enough information to make a fully-informed decision?
Otherwise, you need to go through the normal decision-making process: