Win10 x64, nVidia GTX1080Ti
Broken: All so far
Short description of error
One might not consider this a real bug, but IMHO this can easily destroy (as in "change") your particle simulations during a project so I consider it a showstopper bug.
The value "ParticleSettings.particle_size" can be changed in two different places in the particle settings tab.
And under "Render":
This makes perfect sense if you want to e.g. simulate particles with a certain size but render them as larger or smaller instances (Groups, Objects, ...).
The problem is that both sizes point to the same data path and change each other. So if you use "Fluid" particles and bake your simulation, most of the particle settings UI turns grey and can't be changed (because now the bake is in control). You're still allowed to change how particles get displayed and rendered. And you can also change the render size of the particles BUT as it uses the same data path you don't only change how they're rendered but you now change a setting of the simulation / physics. You can successfully render the baked sim with larger particles if you want BUT as soon as you have to rebake your whole simulation will totally change which is a true production nightmare.
The render/display size could be made totally independent or a factor of the physical size but the display size should NEVER influence the physical size.
This "feature" is currently making an otherwise super smooth project a lot more tedious to handle and I hope it can be "fixed" with some band-aid until the new particle system in 2.8+ comes.
Exact steps for others to reproduce the error