Seconding the comments by Rello and Bdp24, if you were to put a shorting plug on an unused analog output, you would be stressing the circuit which drives that output, by causing it to supply a greater than normal amount of current. Whether or not that would affect long term reliability, as well as functionality and sonics, would depend on the design of the particular circuit, including its output impedance.
Also, if you were to put a shorting plug on an unused second main output of a preamp, since most (but not all) preamps which provide two sets of output jacks simply wire the two sets of jacks directly together inside the rear panel, doing that would kill the signals at the output that is being used. Perhaps that was the reason for HP's problem that Bdp24 referred to.
And depending on the specific design shorting a tape output might also affect the main signal path.
If you want to consider putting something on unused RCA video or digital audio outputs, what should be done is to apply a 75 ohm termination. I don't know if that would make any difference, and I would not expect any difference it may make to be consistent from system to system. 75 ohm terminations are readily available inexpensively in BNC form, since they are widely used in pro applications. But the RCA versions I've seen, which are marketed to audiophiles, tend to be absurdly expensive. If you can't find an RCA version costing say a few dollars, you might consider either making your own, or else using a BNC version (as shown here) together with an adapter (as shown here).
Regards,
-- Al
Also, if you were to put a shorting plug on an unused second main output of a preamp, since most (but not all) preamps which provide two sets of output jacks simply wire the two sets of jacks directly together inside the rear panel, doing that would kill the signals at the output that is being used. Perhaps that was the reason for HP's problem that Bdp24 referred to.
And depending on the specific design shorting a tape output might also affect the main signal path.
If you want to consider putting something on unused RCA video or digital audio outputs, what should be done is to apply a 75 ohm termination. I don't know if that would make any difference, and I would not expect any difference it may make to be consistent from system to system. 75 ohm terminations are readily available inexpensively in BNC form, since they are widely used in pro applications. But the RCA versions I've seen, which are marketed to audiophiles, tend to be absurdly expensive. If you can't find an RCA version costing say a few dollars, you might consider either making your own, or else using a BNC version (as shown here) together with an adapter (as shown here).
Regards,
-- Al