Most compression problems I run into aren't caused by bad tools. They're caused by a handful of habits that seemed perfectly reasonable in the moment. Here are the five I see most often, roughly in order of how often they bite people.
Compressing the same file more than once
Every time a JPEG gets re-saved with lossy compression, it loses a little more information, even if the quality setting stays identical each time. A photo that's been resized, compressed, reopened, cropped, then compressed again has usually gone through several rounds of quiet quality loss that stack up in ways you won't notice until you compare it side by side with the original. Where possible, keep your original file untouched and do all your edits in a single export pass from that original, rather than repeatedly re-saving an already-compressed copy.
Ignoring the actual display size
Compressing a 4000px-wide photo down to a small file size while keeping the full resolution wastes effort. The compression has to work much harder, and sacrifice far more visible quality, to hit a small file size at a huge resolution than it would at a resolution that actually matches where the image gets shown. Resize first, then compress. It's a small habit change with an outsized effect.
One setting for every type of photo
A busy, detailed photo, foliage, crowds, textured fabric, hides compression artifacts fairly well because there's already a lot of visual noise for the eye to sort through. A smooth image, a clear sky, a plain studio background, skin in a close portrait, shows banding and blockiness at the exact same setting that looked perfectly fine on the busy photo. It's worth spot-checking smoother images on their own rather than applying one blanket setting across a whole batch and assuming it'll hold up everywhere.
The same compression setting can look clean on one photo and visibly broken on another. The content of the image matters just as much as the number you typed in.
Reaching for PNG out of habit
This one comes up constantly: someone exports a photo as PNG because "PNG is higher quality," without realizing that's only true in the lossless sense, and photos rarely benefit from lossless storage the way flat-color graphics do. The result is a file three to ten times larger than a JPEG or WebP export that would've looked practically identical on screen.
Trusting a slider instead of a target
Dragging a quality slider to 70% and hoping it lands under your size limit means either overshooting, too big, upload fails, or undershooting, smaller than it needed to be, quality sacrificed for nothing. Tools that search for the highest quality under a specific target size, rather than a fixed percentage, consistently do better because they adapt to what the individual image actually needs instead of guessing once and hoping.
The fix for all five, honestly: keep originals untouched, resize to the real display size before compressing, spot-check smoother images separately from busy ones, pick JPEG or WebP for photos, and aim for a target size rather than a blind quality percentage. ImgReduce handles that last one automatically — you just set the number and let it work.