I've had a rash of problems that all could have been prevented with a little attention to detail.
1. Applicants not meeting the requirements for their certificate/rating they are applying for. For example: Cross country's that are too short distance wise. Short the number of night landings and/or night time. Missing dual cross countries. For private pilot applicants, logging instrument time in an AATD or BATD (the 3 hrs of instrument must be in an airplane). Instrument applicants no shooting 3 different instrument approaches on the long IFR cross country (contact, visual, PAR & ASR approach do NOT count!). Not filing IFR on the cross country. Not doing the long IFR cross country with a CFI-Instrument. And not having 50 hours PIC cross country. For commercial applicants, try to log one flight to meet two requirements (can't double dip), Logging both solo and supervised PIC towards the 10 hour requirement (this must be either 10 hrs solo or 10 supervised PIC, they can't be combinded). And if you don't have an Instrument rating prior to applying for the Commercial Airplane certificate, you mush have 10 hours of Instrument training from a CFI-Instrument.
2. Several applicants for advanced ratings have shown up without a current flight review. (Really, how does this happen?)
3. Missing endorsements. AC 61.65G was released in August of this year. The most commonly missed endorsement is the 61.39 requirement. Remember, There are two parts to that endorsement that need to be signed off. The second most missed endorsement is the 90 day solo endorsement for student pilots.
4. Missing Registration and/or Airworthiness certificate. Remember the AROW acronym? One airplane showed up with a temporary registration certificate that expired almost a year ago.
5. Aircraft not being Airworthy. One showed up without a current Annual. Several have shown up with AD's that have not been complied with. Almost all where because they over-flew a reoccurring AD. One airplane showed up with AD's signed off for a different type of airplane (The nut plate AD and the Rudder stop AD do not apply to the Cessna 172 series). One applicant showed up with fuel gauges that where not working.
All these things are usually check ride show stoppers and in a few cases the airplane is grounded for awhile. All of these things can be prevented with a little diligence from the flight instructor. I'm not that smart, I catch these things with use of a check list. There is no reason you can't develop your own.