unpleasant result e.g. a delayed effect of a drug

consequence, effect

to get or take revenge for (oneself) for (something)

retaliate

something that follows or is caused as the result or effect

result, outcome of action

an effect or result (of an event or of circumstances)

consequence, effect

an amount of money by which a payment may be reduced

refund given to purchaser

surprising because not expected

surprising

unexpected (the usual word)

surprising