In a question on StackOverflow a few days ago, a user was asking how to ensure that a document hasn't changed between when a client read the document and wrote to it. If user A reads the document and makes some changes (through a web form, for instance), the change should be accepted if and only if no other user B has updated the document since when user A read the document.
MongoDB doesn't support transactions, and even if it did, they wouldn't help in this case. An assumption underlying the question is that the time between any given user's reads and writes is long -- otherwise explicit/pessimistic locking would be the simplest solution -- in which case holding a transaction open on a traditional database server would be prohibitively costly (in terms of resource usage and performance).
The solution is to leverage MongoDB's atomic update semantics with an optimistic concurrency solution. This comprises four basic steps: 1. read a document; 2. modify the document (i.e. present it to the user in a web form); 3. validate that the document hasn't changed; 4. commit or abandon the user's update. For anyone who's used a source code version control system before, these steps should be familiar (i.e. pull, work locally, commit, and push for git users).