1. Money stuff - running out of money, taking on work that you know you shouldn't because you need the money, working a day job to pay the bills (to the detriment of the startup) etc.
2. Customer stuff - excessive focus on a handful of big customers (very tempting with a big customer to keep selling them more product); excessive focus on unprofitable customers (even some big customers can actually wind up being completely unprofitable).
3. Internal stuff - hiring too quickly, firing too slowly; poor management; allowing bad culture to creep in; not communicating.
4. Legal and financial stuff - not doing accounting or incorporation by the books (yes, its cheaper to do it on your own, but it will bite you later on); not being careful with expenses; not having proper contracts with customers (handshake works in some industries, but it isn't wise); not enough policy or too much policy!
5. Sales stuff - getting lazy after making a big sale and letting your pipeline dry up! Selling stuff you can't deliver on, or that you know will cause problems later on.
The above is just a taster. There's many many similar potholes, and I've stepped in most of them over the years.