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A ripple effect occurs when a change in one part of a system makes other changes in other parts of the system necessaary.
Bad system design and especially high couplings make the system fragile. A small change in one part of the system breaks existing functionality which is logically unrelated to the change. In order to keep the system working, further changes are necessary. these changes in turn may impose even more changes and so on. The change ripples through the whole system. Small changes have large effects just like throwing a tiny stone into the water creates ripples spreading all over the pond.
Such ripple effects need to be avoided as they increase the effort for making changes. Furthermore they impose plenty of possibilities for introducing defects. Making all the necessary changes correctly and not forgetting some of them is error prone.