I was trying to find ways to crash the game without using glitches or lag or such and I found one. This is a cool test map I made to show off here.
I just used a bunch of monstermakers stacked on top of each other that all spawn a lot of tripmines, spawning too many entities and crashing the game.
Jeffrey's Crasher. Use if you want to make some sort of horrific Kaizo trap in Half-Life.