The bug

If a selector that only has the potential to select one entity (@p, @s, @r, @e[c=1], @a[c=1]) has 4 minus signs, it attempts and fails to parse as a UUID.

How to reproduce

Example command:

/testfor @p[x=-7,y=-3,z=-1,r=0,rxm=-3,c=1]

Which results in:

The entity UUID '@a[x=-7,y=-3,z=-1,r=0,rxm=-3,c=1]' is in an invalid format

Whereas the expected normal error message (for just not finding the entity) would be:

Selector '@a[x=-7,y=-3,z=-1,r=0,rxm=-3,c=1]' found nothing