When do Karnaugh Map terms make no groups?

I understand how K-Maps work and what their utility is, but I can't seem to figure out under what conditions there could be no groups made. (When an expression is not simplifiable further? Won't it always be while speaking of canonical terms?) Do help me out.

An example of a Karnaugh map without simplified or merged blocks:

This example depicts the exclusive OR of four inputs a, b, c and d. No pair of terms can be merged as none of them forms an adjacent couple.

Terms in a Karnaugh map correspond to a conjunctions (logical AND) of positive or inverted inputs. Two terms can be merged to one, if they exactly differ in the polarity of one input.

Example:

Term abc can be merged with term abc'. The resulting term is ab. Input c can be omitted as it occurs with opposite polarity in the two original terms.

ab = ab(c+c') = abc + abc'

Recommendation:

My favorite online site to experiment with Karnaugh maps is here.