Compressed. The compressed stream isn't byte aligned. Normally this isn't a problem because an application will flush the output and pad it to end on a byte boundary before closing a file (see https://www.bolet.org/~pornin/deflate-flush.html for a good explanation)

Where this becomes a problem is when someone thinks "I'll compress my logs as I write them out". The next problem is what to do on restart. It's tempting to think "I can just append to the existing file because I can concatenate two gzip files". But if the restart is after a crash rather than a clean exit you get log salad.