I was using ILM (Identity Lifecycle Manager) to synchronize users cross forest to prepare for cross-forest mailbox moves using the sample code provided by Microsoft. Please see:
I was synchronizing over 5000 users to the new forest. Synchronization was failing with stopped-error-limit.
I found out that MIIS / ILM and FIM only allow up to 5000 objects by default. You can change this with a DWORD in the registry which you create under HKLM\SYSTEM\CurrentControlSet\miisserver\Parameters. This is documented by Microsoft on KB2387673
I set my limit to 10,000 which resolved the problem.