A one-banana problem is an easily resolved issue.
According to the New Hacker's Dictionary, the term originated in the mainframe era. Reportedly, programmers and hardware experts claimed that the jobs of sysops (system operators) could be performed by trained monkeys. Accordingly, the monkeys would be paid in bananas.
At IBM, a conceptual system arose ranking problems, according to their difficulty, as one-, two- or three-banana problems. Beyond the three banana limit, it was considered advisable to request assistance from the manufacturer.
Alternate systems in other environments included grapes as change. At International Computers Limited (ICL), five grapes was equivalent to one banana and the upper limit for in-house tasks was three bananas plus some number of grapes.