The current FCC vanity call sign program has been in place since 1995, yet roughly 20% of vanity applications are dismissed because of errors by the applicant. Some common reasons for dismissal with actual examples are:
Does not meet the FCC guidelines for amateur callsigns - K9TRAINER for example
Reserved or restricted calls - we tend to think of 10 call regions, 0-9, but for the vanity program there are 13. Alaska, Hawaii and Puerto Rico have their own allocation blocks. AL4T is only available if your mailing address is in Alaska.
Applying for a callsign before your upgrade is processed by the FCC. You must be eligible in the FCC ULS for the callsign requested as of the exact moment your vanity application is filed, even though the FCC takes 18-20 days to process applications based on the day of the week they are filed. If you just upgraded - congratulations! Wait until your upgrade is showing before you apply for your new call.
Applying for an expired call too early. Expired callsigns have a 2 year grace period - for the previous holder (or close relative) to apply for reinstatement. Everyone else must wait 2 years + 1 day to apply.
Applying too late for an expired call. All 1x2 calls and 2x1 calls in the 4th, 5th, 6th, 7th and 12th call regions are scarce and get snapped up on the first day they are available - 2 years + 1 day after expiration. Apply on that day to have a chance at them.
Multiple vanity applications on the same day. The FCC permits only one vanity application per day and any other applications filed on the same day will be dismissed. If you discover an error after you click Submit, don't file a new application, just amend your current one.
Slash Zero - If you apply for a callsign with a zero in it and your pc uses anything other than the standard ASCII zero character, the FCC cannot recognize it as a zero and will dismiss the application.
There are hundreds of 1x2 and 2x1 calls available, thousands of 2x2 calls and many thousands of 1x3 calls. If you have an interest in one, maximize your chances for success:
Understand the vanity rules for which format you are eligible for
Research which calls are actually available
Lean the vanity application process and file properly
There are several great sites for vanity call sign information. My personal favorite is by AE7Q. It is well organized, updated daily as the FCC files update and contains the most accurate information on FCC vanity call sign rules.
AE7Q's Amateur Radio Database Query Tools