As mentioned already, "over-stopping" will do no more harm than leaving the print in water (of course, you can't do that for hours...).
Over-fixing is a bit more complicated. Acid or near-neutral pH rapid fixers (ammonium-thiosulfate-based fixers) will start to bleach the least dense areas in a print after a long time in the fix. However, I've purposely overfixed up to 10 minutes and could see no ill effects at that time, so I suspect that the time needed to visibly bleach a print would have to be longer than that. I understand that alkaline fixers do not have this problem.
The thing to be careful of is wash time. As bence8810 mentions above, longer fixing times can require longer wash times for fiber-base prints. This is especially true if you use the Ilford archival sequence, which uses "film-strength" rapid fixer for only one minute. The idea here is that the short fixing time keeps the fixer from soaking into the paper base and saturating the paper fibers. The shorter wash times with this regime result from only needing to wash fixer and byproducts out of the emulsion layer, not the paper base. If you fix a fiber-base print for much longer than that 60 seconds, you will saturate the paper base and need a suitably longer fixing time. Furthermore, if you leave fiber-base prints in the fix for much longer than optimum, especially if the fixer is approaching its capacity, complex compounds can adhere to the paper fibers that are harder to wash out, requiring even longer times.
Personally, I don't like or use the Ilford sequence; I use a two-bath fixing regime in "paper-strength" rapid fixer, fix for 1.5-2 minutes in each bath and wash appropriately longer. For any work-flow with fiber-base prints, a wash-aid (Hypo Clearing Agent or the like) step is crucial to getting a good wash.
Best,
Doremus