Buying old used gear off second market is always gamble. History of use is unknown, and who knows how well owners cared the instrument.

Problem get more difficult as self testing and diagnostic routines are not capable to cover all possible functions, specially for complex meter like HP 3458A with hundreds of componenets. Some of hidden failures can be revealed after longterm testing, others can remain hidden and show up after years.

Similar happen to our HP 3458A restoration project, after repair on low current DCI ranges I had SELFTEST pass no less than 10 times, run ACAL ALL few times and even CAL 10V and 10Kohm. Then meter was left running for 5 hours, sampling 1Meg resistor, after which measurements were halted, with ERROR 114 — multislope rundown convergence. Now it doing this error every second in random intervals, if sampling speed slower than 100uS used.

More details and repair worklog linked here