This is a stupid situation, but it happens sometimes. Say you have "if" block (in logic), which goes to one or several actions, and then should return to normal flow. But than you accidentally put the arrow so that it goes back to "if". Later when you notice problems you can spend hours trying to figure out what is wrong, depending on the side effects this creates. Debugger doesn't always help - I've been in the situation when there was DB update inside of one of nested actions, and debugger was just stuck on it, so it was not clear where is the problem.
Off course this is human factor, but would be great if Service Studio noticed such infinite loops and gave a warning about it. As a simple way, It might warn on any flow which returns to original "if", or, a little more complex - just the one that doesn't have modifications of the variables used in "if" (since there is no thing like "break" in Outsystems, there are only 2 ways of exiting such kind of loop: a change of "if" result and exception).