The grouping-technique mistakes — missed wraparound, undersized groups, invalid shapes — are covered separately in Common Karnaugh Map Mistakes. These are different: process and notation errors that lose marks even when your actual grouping technique is flawless.
Swapping minterm and maxterm notation under pressure
Σm and ΠM look similar enough at a glance that misreading one for the other under time pressure is a genuinely common slip — and it doesn't just give a wrong answer, it gives an answer to the complement of the intended function. If a final answer looks unexpectedly complex or unexpectedly simple compared to what the problem seems to call for, re-checking which notation the question actually used is worth the ten seconds it takes.
Answering in the wrong form entirely
A question that explicitly asks for POS but receives a correctly-minimized SOP answer (or vice versa) often gets marked wrong regardless of whether the underlying simplification was done correctly — the grader is checking for a specific form, not just a correct truth table match. See SOP vs. POS Explained to be sure which is actually being asked for before starting.
Inconsistent variable ordering between the problem and your map
Minterm numbering depends entirely on which variable is treated as most significant. Take F(A,B,C) = Σm(1,2,3) with the standard A-most-significant convention:
Accidentally reversing which variable is "first" (treating C as most significant instead of A) relabels those same three physical rows as Σm(4,2,6) instead — a completely different-looking, differently-numbered problem:
Both are internally consistent, correctly-minimized answers — to two different labelings of the same physical truth table. The fix is mechanical: fix the variable order at the very start (usually whatever order the problem lists them in) and never switch mid-problem.
Attempting a manual map on too many variables under time pressure
Past 4 variables, manual grouping error rates climb fast under time pressure specifically, even for someone who handles 4-variable problems comfortably with time to think — see Manual K-Map Simplification vs. Software Tools for why. If a timed exam includes a 5- or 6-variable problem, budgeting extra time for the cross-tile checks covered in Reading 5- and 6-Variable Multi-Tile Karnaugh Maps is worth planning for in advance, not discovering mid-exam.
Never checking the final answer against the original truth table
The single fastest error-catching habit: pick two or three rows from the original truth table (including at least one 0-row) and verify the final simplified expression actually produces the correct output for each, before moving on. It takes under a minute and catches a surprising share of the mistakes above, including notation swaps and variable-order slips, since both produce an expression that simply doesn't match the original table when spot-checked.
Practice catching these before the exam: work through the practice problems, or verify your own worked answers against the solver.