Typically, a zap can end up in a loop if it has an action step that will do the same thing that the zap's trigger is watching for.
For example, if a zap is triggered by watching for an update on a Google Sheet, but the zap in turn updates the same Google Sheet, this can cause a loop because the zap will trigger repeatedly without stopping.
Looping can also happen across multiple zaps. Zap A could have an actin step that triggers Zap B, which in turn could have an action step that triggers Zap A again. In this situation you'll get a loop.
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If you notice a zap is looping, then the quickest way to stop the problem is to switch the zap off from the Zapier Dashboard. Then don't switch it back on until you're sure you've fixed the issue.
Your best tool for avoiding loops with your zap is the Filter step. You should be able to construct a set of rules to stop the zap running in situations where you don't want it to, and break any loops.