Solvents/reagents that cause liquid transfer problems
BSA (Bovine Serum Albumin), at most 10% concentration
Glycerol, at most 50% concentration
DMSO, at most 50% concentration
Soaps/detergents, at most 10% concentration (or 100% for cleaning cycles)
All of the above can either be too viscous to pipette accurately or froth/foam when used in concentrations greater than those listed above. If you absolutely need to use in excess of these guidelines, validate first!