The problem is not that a file with a lossless extension (e.g. PNG) may have actually had lossy processing.
The problem is that it's not necessarily trivial to tell programs/pipelines/transforms to act only losslessly (there's definitely no common switch; Quality 100 is in fact still lossy in some encoders/formats). In practice some programs use the extension to realize what they should be doing.
The problem is that it's not necessarily trivial to tell programs/pipelines/transforms to act only losslessly (there's definitely no common switch; Quality 100 is in fact still lossy in some encoders/formats). In practice some programs use the extension to realize what they should be doing.