Which graphics clash and which graphics don't?

One of the easiest ways to wreck the aesthetics of your game (unless it's Normal Super Mario Bros) is to use graphics that clash horribly. There are cases of obvious graphic clashes such as using original SMB goombas with a M&L3 Mario, but sometimes it is possible to mix and match two different styles without major clashes. The question is, which styles go together well and which styles should never be used in the same game? This applies not only to the original sprites used in the games, but also edits and custom graphics as there are seldom any custom styles that have all the objects that SMB3 or SMW or YI has.

Sprites clash when their styles don't fit.
Some styles would be: ML:SS, SMB3, SMW, NSMB and YI.
You can't make a game mixing all these styles.
Some are outlined, and each one is shaded differently (3D, dithered, cartoony, simple...)
PLUS they all have totally different colors.
You could look up for sprites from different games that fit in style and recolor them.
Original colors and a not-too-big palette for the whole game make everything better.

Graphics clash when styles do not match. Black Squirrel's tiles go good with SMB3 because they were designed to look like SMB3.

The keys to visual appeal are a palette, and a consistent style guideline. Even if there is a tiny element of the style that is missed, something won't feel right, and it isn't as visually appealing.

In this way therefore, it would appear impossible to have sprites from any combination of games put together in one game without clashing.

Which is essentially true.

Different angled sprites clash badly.

That's why I hate when people use M+L sprites for platformers.

Most (if not all) good rules have exceptions. Are there any exceptions in this matter?

Unless it's a group of games where the style essentially remains the same (See: Mega Man 1-6 and 9), or customized/edited sprites designed to mimic the style of a particular game, I'd avoid it whenever possible.

When styles are the same, it's fine. Mega Man 9 sprites are not 100% NES styled, because they do not have the same palette. However, they are in the same exact style, so they fit.

As long as the light source, angle, shading, and palette are similar enough to be considered the same style, then they match.

This is a good example of sprite clashing

college humor so funneh xD xD

Yeah

And they didn't give credit

Yoshi's Island DS

Some of the enemies clash with everything else