From The New-York Times July 9, 1861:
…WASHINGTON, Monday, July 8 …
MUTINY AMONG THE GARIBALDIANS.
It is reported that a mutiny broke out among the Garibaldi Guard to-day. One hundred of them left the camp and came to the city. They were stopped on this side of Long Bridge by the guard, and held there. The military officers on this side will make a word and a blow, and fire on the mutineers, unless they surrender and return to duty. The trouble grows out of some disaffection about the arms given to the regiment. …
So four days after the Garibaldi Guard paraded for President Lincoln, General Scott, and others in Washington, trouble erupted in Alexandria. I don’t know what it means to “make a word and a blow”, but firing on the mutineers is pretty straight forward. Here’s more from New York State Military Museum about the Garibaldi Guard (the 39th Infantry Regiment):
It was mustered into the U. S. service at New York, May 28, 1861, for three years and left the state for Washington on the same day. Camp Grinnell was established near Alexandria and occupied until July 17, when the 39th participated in the movement of the army toward Manassas with the 1st brigade, 5th division, though in the battle of Bull Run the regiment was but slightly engaged. After a few weeks at Alexandria much ill feeling prevailed over the failure to receive some expected privileges and 50 members of Co. G mutinied, but returned to the command after being disciplined by arrest and imprisonment.
The Library of Congress dates the drawing of the Garibaldians surrendering as June 8, 1861. I have not seen any information about trouble with the 39th on that date.