This entry isn't going to be about a specific rule or scenario, but rather just a generalization that I encounter often.
When creating a map, you can have something working, and what you consider a small change, can often cause you to end up rewriting a good portion of your map.
This seems to be especially true when you inherit a map from someone else, either a co-worker or someone that held the position prior to you.
A recent situation I ran into, the map had a rule 3 nested groups deep writing to some temps. They no longer wanted the data for a specific qualifier and threw in a:
If #Field != "value" Then
rule around the statements. When they had the system date on one of the fields linked across, it'd give all the output along with just the date for the iterations that had that qualifier. If they included the date within the IF, moving it to another temp, they'd lose data (because it'd cause an empty iteration).
In the end, they had to restructure the temp and output side of the map, re-write the rules, add indexes etc.
It's easy to get caught up with just making small changes and focusing on what was working.
But you often need to take a step back and look at the big picture again. Once I pointed out what was wrong (different now) they were able to see it as well and run with it.
Which also leads to another suggestion, when getting another set of eyes, or asking for help, start off with the bigger picture of what you're trying to do now. Then narrow it down to what was working before. Explaining it that way, sometimes helps yourself see what's going on as well. If you start with what was working, you're going to lead the person down the same path you took, instead of letting them make their own observations.
In general, just remember to take a step back whenever you get stuck.