miscompute
verb
mis·com·pute ˌmis-kəm-ˈpyüt 
miscomputed; miscomputing
transitive + intransitive
: to make a mistake in computing : to compute wrongly
Overall, the IRS wound up sending nearly 197,000 notices recently to seniors explaining that they had miscomputed their taxable amounts of Social Security benefits …
Neil Downing
Neil Downing miscomputation
ˌmis-ˌkäm-pyu̇-ˈtā-shən
noun -pyü-
plural miscomputations
an error caused by miscomputation
Word History
First Known Use
1637, in the meaning defined above