Stacking Offset - why Edit and Play can look so different
Stacking notes directly onto each other looks vastly different in editor and play mode, and many people seem to forget about this: As a rule of thumb, every note you stack onto another will appear 4|4 pixels more to the top-left corner of the editor, which equals one grid (size 4), so take this into consideration when you're mapping with structure and so forths, as this will result in your pattern being uneven and feeling offset.
As an example, you see that here (in the first image), there are three objects stacked on top of each other - while this looks like they are on the exact same spot in edit, effectively, it will look different in play mode (second image), so keep an eye for these things, especially if you're trying to make symmetric patterns and the likes, as the player will have to start hitting the stack at a position of x|y-(n*-4|-4), so in general, it may work better for you to use tighly spaced notes instead of absolute stacks if you want to have things aligned properly.
Care: This is significantly more important if you have notes spaced close to each other, such as in the following example.
You see that there are are about 3 grids size 4 of space between this sliderend and the stacked triple - the reason behind this is simple: I wanted to have the triple close to the sliderend (like about one grid size 4, so that it barely does not touch), but stacking it there directly would result in an ugly overlap; effectively, the triple will look fairly different in play than in edit (second example).
The exact stacking value varies with CS but it stays in the same relation to your circles; I don't really know exact values though, so always check that for yourself when using exotic sizes (which basically are not 3, 4 or 5).