You're right, I should have said "usually inherent". But it's a major reason for using quaternions. While you stress the case of gimbal lock occuring within a

You're right, I should have said "usually inherent". But it's a major reason for using quaternions. While you stress the case of gimbal lock occuring within a single rotation, which is typically due to the use of Euler angles, there is also the problem of gimbal lock emerging through the composition of two or more rotations in matrix form. I'm vague on this point but AFAIK this problem can arise with rotation matrices whether or not Euler angles are used. Choose three successive unlucky rotation axes and you've got the problem. This seems to get so complicated. See e.g. https://en.m.wikipedia.org/wiki/Davenport_chained_rotations https://en.m.wikipedia.org/wiki/Davenport_chained_rotations