This happened to me once. Jumped into a system... in the middle of multiple pirate fleets on their way out, and they pounced on my fleet before I could get out. Since my endgame fleet was too big, I could not disengage, and I had no choice but to fight the 300+ enemy ship pileup. (Rather than reload, I fought them because it was amusing.) Lost all of my warships, but not before taking out all of theirs. Then it became a standoff between civilians. They just would not retreat their civilians, and we had to deploy. What happened was they deploy a bunch of civilians, then retreat. Eventually, they lost due to running out of CR and self-destructing. After many deploy-and-retreat fake outs, they finally tried to disengage.
It would be nice to know how powerful the enemies are on the other side. With an endgame fleet, nearly all enemies detected are meaningless since your super fleet can effortlessly destroy whatever is on the other hand, but the multi-hundred ship pileup is an exception. But the warning does not tell you if the enemy is a small fleet that you can rollover or a gigantic multi-fleet pileup.