The biggest mistake in Lebanon was made the Marine commander that bunched all those Marines together and left them barely protected. Incredibly stupid. One of the first rules of a Marine, or Army, combat unit, no matter how big or small, is to never bunch up so that one mortar, one grenade, one of whatever the enemy is tossing around will get all of you. If you're vulnerable to attack that is the first thing any NCO, or officer, will correct if they observe that the rule isn't being obeyed. The first thing.
Lebanon: While leaving Lebanon became grist for bin Ladens mill, there was no reason to stay really. This was 1983, there was no Al Qaeda, bin Laden was either working for us or about to, the Gulf War was years away, and so forth.
Further, bin Laden was dead wrong. Most of the world was reading us wrong starting from our exit from Vietnam. Sure, we left Vietnam, but we had stayed for a very long time, and spent a lot of blood and treasure. No other military in the world could have spent that much time, at the end of an equally long supply line, with constantly dwindling civilian support, and no palpable reason for the soldiers to go on, and put up that kind of a fight. They talk about how tough the VC were, or how tough the Mujahideen are, please, they fight on their home turf, or with religious convection, while we fight dangling from the end of a rope.
So, sure you can make the case that leaving Lebanon was a mistake. You could also make the case that they took the bait, whether the hook was purposely baited or not.
BSB