What are the potential negative consequences of miscalculating drug dosages?

Anything is possible: There is no good answer to such a broad question, but anything is possible when a person gets an incorrect dose of a drug, ranging from absolutely nothing bad happening, to the occasional death.

Varies: Each drug has a "therapeutic window" or zone where any dose is likely to do the job intended and not result in significant side effects.In drugs with a narrow window, precise dosing is important, when it is wide, not so much. An overdose of some chemo drugs may kill you, while a double dose of Claritin (loratadine) will do nothing.