As the name suggests, every transaction involves two equal and opposite entries. One entry is a Debit (Dr) and the other a Credit (Cr); hence the term, "Double-Entry".
For each double-entry (also called transaction) the Debits (Drs) must equal the Credits (Crs), which provides a built-in error check.
The only two errors that can be made when using a Double-Entry system are either one of misclassification or omission. A misclassification could occur if a Dr or Cr is made into the wrong account. An omission could occur if you forget to make an entry.