What is says on the tin.
I do appreciate that Unity believes my super uber new Particle system deserves somewhere just north of 15 Exabytes of memory (Seriously I can’t even type that number into windows calculator, I’m 4 digits short, wtf), but considering my laptop is not made in year 2516, I am a bit at a loss here.
How do I prevent this from happening, what am I doing wrong? In the screenshot, I am trying to attach “Particle charge trail” (a child of currently selected object in hierarchy) to the first “Birth” slot of the currently selected particle system - you can even see the stuck messagebox where I accidentally hovered over the collision slot before dropping it in correct one, resulting in editor crash.
I am unable to make a bug report as the Unity instantly decides it needs my entire Project folder, which is somewhere north of 800 MB, and if I manage to upload 20MB RAR file through the entire day on my internet connection, I consider myself a very lucky man that day.
Is this happening only after you’ve upgraded to 5.4?

Happens to me as well about every 5 minutes of work. I’m working with particles too, but no specific steps to reproduce, always at different moments. Can just select a gameObject with particles and get a crash. version 5.4.0p1. Made a bug for it http://fogbugz.unity3d.com/default.asp?822356_e7hppisnrs0shanc. I know it’s not very descriptive, but those crashes are really random and I’m not doing any black magic with particles like scripting or allocating billions of them, just changing some properties in the Editor.
UPD: and yes, that happens only after upgrade to 5.4

Same here took me a while to narrow down what was causing the crash as it would happen about 5-10 min in to working with the particle system. But was able to pin the cause to when messing with the SubEmitters parameters in the Particle System inspector, specifically Collision in my case.
Not only is the crash quite troublesome but its forcing unity to re-import my assets when I load the project again and that’s another 10-20 min of it doing that…
And yes I am working in Unity 5.4
Ouch… that doesn’t sound good.
At least I didn’t have to reimport my stuff.
I thought I replied to this thread, but anyway, temporary fix for me was to un-parent the “subemitter” particle system first.
Adding it as the subemitter system in a parent system triggers a question “Do you wish to re-parent it” or something like that.
Answer “No”, and then manually re-parent it after it was assigned.
This avoids the crash for some reason.