They can! This is quite common, actually, and we have a couple of suggestions in case this does happen.
When a pledge fails to complete as a transaction (once the campaign owner chooses to charge all supporters, it’s usually due to a credit card either expiring in the time between when the pledge was made and the transactions triggered or there not being enough credit/funds in the account charged.
It’s hard for campaign owners to prevent this. One way the platforms mitigate failed pledges is by limiting the time allotted for all-or-nothing campaigns to 60 days maximum. Any longer, and the risk of failed transactions rises significantly.
We also suggest campaign owners build in a 10% buffer into their final goal to account for the possibility of failed transactions. For example, if your goal is $5000, consider a goal of $5500, as the trend shows 10% of pledges are likely to not go through.