Strange behavior: NE555 + 7493 Ripple Counter + 4 to 16 Decoder
The output of a ripple counter does not change from one 'count' to another instantaneously. The new state 'ripples' across the outputs until it has reached the new state.

It is possible that these 'in between' states show up on the outputs of the 4 to 16 decoder as 'random' outputs.

If you clock the ripple counter manually and then look at the outputs they will be correct, because the ripple happens quickly, but the 4 to 16 decoder is fast enough to change according to the in between results.