It is in larger scales that the problem more notably presents itself.
When I was younger, mixing larger scales for armor was very common place. 1/35 scale was just coming into its own, and the Japanese firm of Tamiya was beginning to flood the market. Meanwhile, older models in 1/32 scale were still available, especially with Monogram having made a large number of them (notably their Panzer Mk.IV variants). Most of these kits contained not only armor but figures. Complicating this was Nichimo, which had produced a few kits in 1/30, and likewise some of these kits contained figures.
In due time, it would be relatively easy for a modeller to amass a good many figures. The problem was, of course, that they really varied in size.
Ironically, they still fell within 10% of the median scale, 1/32. A 6' tall human stands 2.25" in 1/32. In 1/35, they stand 2.05", and in 1/30 2.4". In other words, the 2.05" figure would stand 5' 5.6" while the tower over the others at 6' 4.8". Between the extremes is almost a scale foot of difference.
That's not to say that such differences don't occur in nature; they do. But so many other aspects would vary as well; guns, helmets, gear. Ironically, there are actually more figures available in 1/32 (54mm) than in 1/35 (50mm).
The trick is trying to decide how to proceed. Obviously, larger equipment, such as artillery and vehicles, is not so big a deal during play. Even differing scale figures could still be used as long as they were not used together, provided the 10% rule is followed.