File this under things that are obvious once you know it...
If you've done plenty of maintenance work on MakerBot and FlashForge printer hotends, there's a good chance you've also accidentally chipped a fan blade off a cooling fan, when the allen wrench or hex driver slipped when removing or installing the extruder stepper motor. It's a simple mistake to make. So much so that it's even inspired this replacement warning sticker for the extruder carriage:
Well, I've done my fair share of breaking fans, and finally realized this one simple "weird" trick when working on a still-hot hotend: rest your thumb on the spinning fan momentarily while you quickly loosen/tighten the motor mounting screws. It's a lot faster to work that way than trying to carefully aligny our tool without hitting the spinning fan, or to wait for the hotend to cool down.
If you do end up chipping your hot-end fan and need to replace it, do yourself a favor and buy decent ones with enough airflow. Some cheap ebay fans have a much lower airflow and will not sufficiently cool your hotend. When that happens, you're far more likely to encounter filament jams due to heat creep (especially with PLA). For 24V machines (Replicator/Creator line), I particularly like the ebm-papst fan available via DigiKey p/n 381-2367-ND. It's slightly noisier, but it's a rock solid performer.