Given the number of startups I've been a part of either as a consultant or executive, I'd have to say the number of presentations to venture capital and private equity firms now numbers in the hundreds.  Part of my business is helping prepare mgt to present their businesses successfully - so I've seen many common mistakes.  The first one is easy to catch - revenue models that can't be sustained.  In this case I will help a company understand that fact, and many times we can come up with a solid alternative.  But for those with solid revenue models based on an already growing customer base, I've seen deals go sideways in the due diligence process for a different but consistent reason:  The management team is viewed as lacking a solid understanding of their cost structure. 

Note the way I've worded the sentence - they are "viewed as lacking a solid understanding of their cost structure".   Now, these are really smart people and all of the presentations include a proforma which projects revenues and expenses - the usual hockey stick that VCs expect to see (and privately roll their eyes at).   But often times when a VC digs under the numbers, the detail of the costs isn't fine enough, or frankly not believable.  Headcount is usually too low to get the job done, and accounting for expenses related to the business are either too complex and not put into a form they can grasp well (if it an unusual business), or put into "buckets" for a rolled-up income statement and therefore not easily analyzed from a bottom's up approach. 

When a VC is serious about your company, they will build their own forecasts and sensitivity analysis - they will not use yours.  To help convey that you know your business, you must have sufficient expense detail to allow them to build their forecasts with some modicum of comprehension of how you reached your numbers so you can have an apples to apples conversation of cost structure.

Usually in an early-stage startup there are a lot of unknowns - This is not the issue.   The test is in the ability of management to show their leadership in predicting solid scenarios with sufficient detail to show that the mgt team can really manage in bad times.   You're only making their life harder if you don't take a more realistic view and show sensitivities that include worst-case scenarios with the detail to support that they are indeed worst case. 

For example, do not just pool headcount numbers - have a detailed headcount matrix showing the timing of adding each position.  Show the exact ratios of customers to customer service reps.  How many calls per day do you believe they can handle?  Not being able to explain this shows a lack of understanding of your business.  Be careful of a top-heavy exec team with no middle mgt as span of control becomes an issue that needs to be mitigated for a tightly managed company going through fast-paced growth, so be reasonable.  Interestingly the product development teams are generally pretty light - it's as if CEOs are planning for their products to be perfect, with no bugs, new feature ads, etc. - like that really happens.

I've seen very comprehensive revenue models detailing customer growth sensitivity, but much less effort seems to go into detailing cost projections.  If this is you, the VCs may indeed still invest in the company upon conducting their own due diligence - but with the decision made that the company will need more experienced leadership to pull it off.  The problem is, you won't know about this before you hand over part-ownership of your company.