Heard this one third-hand, so it might be Urban Myth.
The Chinook had a Relief Tube under the right-hand seat. Occupant of that seat needed a wee, but didn't want to do it sitting down, so he pulled the tube out to the left side, unstrapped and stood up, and started to have a wee while standing between the seats.
Other pilot needs to reduce power, so lowers the Thrust Brake (collective) which pinches the relief tube between the lever and the seat. Pilot wonders why the lever won't go down any further, and copilot sees that the tube is full and backed up to overflowing, with the flow still flowing. Pilot calls to copilot to get back in his seat, as he thinks there is a control problem. Copilot clenches off the flow and re-zips, but has a full tube in his hands.
Pilot says "SIT DOWN!" so copilot releases the tube, to much splashing of product on self and aircraft. Resumes seat, asks pilot what the problem is, and both then realise what had happened.