I have a trusty Wages application that prints wage envelopes and calculates levies payable, loans by employees, etc.  It is several years old, now running on Sesame 2.6.4. Last week it suddenly calculated wage amounts with small errors, e.g. R199,99 instead of R200 (South African Rands).

On the pay form, the wage/hour is rendered as Number, not money, because it often needs up to six decimal places when, for instance, laws regarding work hours permissible are changed.  I set this in Regional Settings on the OS: money as 2 digits, number as 6.  I discovered that the latest Win 10 Update (over which, of course, I have no control) had, in Regional Settings, arbitrarily altered the number of decimal digits in Number from 6 to 2. 

It didn't merely show the 2 digits while retaining the number entered.  Instead, it totally discarded the last 4, introducing the error!  On changing the Number setting on Regional Settings back to 6 and re-entering the wage/hour amount, the problem vanished.
If anyone else has this problem, check your regional settings!