One only has to compare an aldehyde fixed mitochondrion with a quick frozen one. In quick frozen tissues, mitos tend to be smaller, much more condensed. In my own work with cholinergic nerve terminals in torpedine rays, we showed that aldehyde fixed nerve terminals had no vesicles along the pre-synaptic membrane but quick frozen tissues had a zillion. Aldehyde fixation caused calcium entry and loss of active zone vesicles. Microtubules are lost if one fixes at 4 C but not if you fix at room temp.
Nuclear volume varies significantly depending on the fix and dehydration protocol.
There are lots of other examples. Lots of times this has happened but my life is a bit too busy to go into much detail. Look at the mesosomes in bacteria, there were actually fixation artifact, found when free fracture was done. The pore complex in basidiomycetes and ascomycetes in fungi. The cell membrane in bacteria often is artifactual shown true by free fracture where there is no fixation.