What do I do if I accidentally placed a trade in the wrong account?

TRANSFER POSITIONS INTERNALLY

If you executed a trade but you selected the wrong account by mistake, you can either trade out of the position or request an internal transfer to your other IBKR account. Please note, you will only have the option in Client Portal to internally transfer positions if the account holder information is identical and the receiving account has approved trading permissions for the position. If the choice to internally transfer positions is not available, than those criteria were not met. For example, you would not be eligible to internally transfer positions from a taxable Margin account to a non-taxable IRA account. 

Additional Information

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Can I transfer my positions to another party?
Is it possible to set up a recurring internal transfer between IBKR accounts?
How long does it take for an internal transfer of funds?