What's the most common MISTAKE newbies make while sharpening a chainsaw chain?

[Okie]

This is also not mentioned in the instructions. It's a learning experience. The more you file the duller the chain gets if you do not pay attention to the rakers.
using one of these type file guides and having the wrong size file in the guide and also using one of these type guides and NOT setting the raker depth BEFORE using the guide. (if rakers are too high the file is too high on the tooth when the tooth is worn about 1/2 way back) File is actually dulling the tooth as you file.

The correct size file is usually not marked on the metal guide, just on the paper package.
Oregon 25894 5/32" 4mm Chain Saw Blade File Guide

 

[rtrsam]

Quick glance through the entire thread, I swear this topic comes up every few months...

Mistakes I've made:
Try to file with the wrong size file (I knew it was wrong, it was what I had, 3/16" file, needed 7/32": had to keep a lot of upward pressure on the file to avoid hooking the tooth.
Try to file without a handle: that tang is a pretty sharp little point, even with a rag over it it punches through into the hand.

Mistakes I've seen others make:
Backslope top cutter: not enough down pressure into the tooth (using the right size file, they just let the file ride too high, then it magnifies.
Blaming the rakers (depth gauges) or a "bent bar" (or "bent blade") when the saw cuts crooked.
Use only the center section of the file: the ends of the file are basically brand new but the center section is worn out from short strokes. You paid for the entire file, might as well use the whole thing.
Occasionally rotate the file and knock the filings off it while using it. I'm pretty casual about this but it needs to happen.
File too deep: hook the tooth and start to cut into the top of the drive link. every saw chain I've seen break had been filed like that and the break occurred at a filed drive link.

 

[Hermio]

View attachment 1250227
Here’s one. STIHL and Oregon also used to sell them.

Philbert

You should see the saw that goes on!

 

[pdqdl]

I find with chainsaw stuff a lot of people claim the way they've been doing it for years is the only way to do something.

So true.
Let's not limit that comment to just chainsaw stuff. I'd say that applies to everything humans do. I'd guess that even when folks are advising others how to be more pious, their own method is obviously the best of all.