Some people recently discovered a bug in the Unlink XMR mixer that may have caused some transactions to become stuck.  If you recently had a transaction where your funds did not arrive, please visit unlink and click the "Check Status" page and put the encrypted data shown at the beginning of your transaction in the the large text box and click "Recover Transaction."  If this does not resolve the stuck transaction.
For full transparency, the bug was during the creation of a transfer using the monero wallet API.  In cases with a more favorable exchange rate, the extra coin was being sent to the next wallet as a separate destination/amount pair from the expected amount.  For example, given a transaction of 1.0 xmr (expected) and getting 1.02 xmr in the exchange, we expect that 1.0 gets forwarded, 0.015 goes toward the fee, and then the extra 0.005 also gets forwarded.  The buggy code would create a transaction with those 3 amount/destinations.  This was marked as an error in the monero client and the transaction would not complete.  The fix is to group the amounts by destination, resulting in something like forwarding 1.005 to the next wallet, and 0.015 to the fee wallet with only those 2 amounts specified.