What are some common mistakes in Sandwich Sudoku?

The single most common error: including the 1 or the 9 in the clue sum. The clue counts only the digits strictly between them - The bread is never part of the filling. A related slip is building candidate combinations that include 1 or 9: a three-cell sandwich summing to 12 can never be {1,2,9}, because both of those digits already serve as the boundaries of the line's sandwich. Every combination you write down must come exclusively from {2,3,4,5,6,7,8}. If a deduction suddenly seems too easy, check whether you quietly let a 1 or 9 into the sandwich.

A clue of 12 does not mean two cells, or three - It means any length whose range contains 12 (two cells: 5-15; three cells: 9-21; even one cell is excluded only because 12 > 8). Until the 1 and 9 are actually located, hold all consistent lengths open and use the length ranges to enumerate them. Solvers who commit early to one length tend to discover the contradiction twenty placements later, when it is expensive to unwind. The clue constrains the geometry; it does not decide it.

Two omissions cost the most time. First, deprioritising the extreme clues: 0 and 35 (and near-extremes like 2-4 and 33) should always be processed before anything else, because they fix geometry instantly. Second, forgetting the outside of the sandwich: the digits outside always sum to 35 − clue, and with large clues that outside total is tiny and brutally constraining. Re-scan both sides of every clue line after each placement - The line you solved half of often finishes itself.