To make things clear, here is output from my test program (three threads: main, client, sender, neither client nor sender has event loop, client and sender live in main thread, socket lives in client thread):

Qt::AutoConnection (slot is executed in main thread):
Qt Code: 
  1. "main,                thread: b70a28e0"
  2. server is listening
  3. incoming connection
  4. "client started,      thread: b6e82bb0"
  5. "sender started,      thread: b6681bb0"
  6. "sender emit signal,  thread: b6681bb0"
  7. "client slot,         thread: b70a28e0"
  8. QObject: Cannot create children for a parent that is in a different thread.
  9. "sender emit signal,  thread: b6681bb0"
  10. "client slot,         thread: b70a28e0"
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Qt::DirectConnection (slot is executed in sender thread):
Qt Code: 
  1. "main,                thread: b71698e0"
  2. server is listening
  3. incoming connection
  4. "client started,      thread: b6f49bb0"
  5. "sender started,      thread: b6748bb0"
  6. "sender emit signal,  thread: b6748bb0"
  7. "client slot,         thread: b6748bb0"
  8. QObject: Cannot create children for a parent that is in a different thread.
  9. "sender emit signal,  thread: b6748bb0"
  10. "client slot,         thread: b6748bb0"
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Qt::QueuedConnection (slot is executed in main thread):
Qt Code: 
  1. "main,                thread: b70df8e0"
  2. server is listening
  3. incoming connection
  4. "client started,      thread: b6ebfbb0"
  5. "sender started,      thread: b66bebb0"
  6. "sender emit signal,  thread: b66bebb0"
  7. "client slot,         thread: b70df8e0"
  8. QObject: Cannot create children for a parent that is in a different thread.
  9. "sender emit signal,  thread: b66bebb0"
  10. "client slot,         thread: b70df8e0"
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