This most often happens when the constituent's CVV (the security code on the back of Visas and Master Cards and the front of Amex cards) is incorrectly entered by the donor. When this happens an authorization is acquired and then voided within moments. The banks involved may process the authorization immediately but then take days to process the void. This can appear to be a charge on the card if the constituent checks their bank online. Once the void is processed, the authorization will disappear.

To determine if this is the issue, run a decline report (instructions here). If the error is something like "Voiding transaction due to CVV2 failure," than indeed this is the issue.