In addition to the adjustment type in NL7's answer, there are a host of others. If there are any adjustments, form 8949 is required, if not, the gains can be separated into short and long-term and added together to be entered on Schedule D. Anything requiring an adjustment code in column F of the 8949 requires an entry in column G.
Some other example entries for column F include:
B Basis is reported incorrectly on the 1099-B (as mentioned above)
T Whether the gain is Short- or Long-term is reported incorrectly on the 1099-B
W Whether the sale of the security was considered a wash sale**
N Whether you received the amount as a nominee
(see the 8949 instructions for a complete list)
**A wash sale occurs when you sell or trade stock or securities at a loss and within 30 days before or after the sale you:
Buy substantially identical stock or securities,
Acquire substantially identical stock or securities in a fully taxable trade,
Acquire a contract or option to buy substantially identical stock or securities, or
Acquire substantially identical stock for your individual retirement account (IRA) or Roth IRA.