I suggest wild headcrabs breaching into the sewers.
I don't mean NPC headcrabs walking around, just some players that have the skin of one.
I suggest it would be a minor breach, because bigger numbers of headcrabs might make organising such event very hard.
And NO, headcrabs would not walk around in P1, and if this would happen, CCA would rape them take them all out.
I think P3 and the sewers would be the most damaged by this, chaos would run through the unsafe streets of third precint and the sewers.
It would even include some fun for the citizens, because they would have the most problem, because they are the defenseless.
Let me write a little intro :
You walk down the street of the Third Precint, it's quiet, nobody's around.
But something is not right, the silence breaks, as you hear muffled scratching under your feet.
There's something in the sewers...
You focus on it, and you realise, that these scratching can be... Headcrabs
Nothing good will come out of this, you better get back into the safety of First Precint.
Watch your back.