1 min lesson
When Tab is quiet or wrong
Put yourself in this case: "The edit looks plausible. Which warning sign do you check first?" Give the clearest next step.
Step 1 of 3
A plausible suggestion can still hide this problem: Reporting 'Tab is bad' without a file, language or repro.
Interactive diagram. Tab through its regions; each focused region shows its detail in the panel below.
diagram: compare
Pick a row to see what to look for.