1. Using too light or too dark of a color for the subtitles. Many times it blends into the background making it nearly impossible to read. E.g. - black subtitles where scenes are generally nighttime or white subtitles when most scenes take place outside in bright light. (whoops!)
2. "Flash" subtitles. This is when the subtitle is only on screen for less than 1 seconds. (maybe they were trying to save money)
3. Inaccurate subtitling. Some subtitles aren't accurately translated. E.g. The Spanish word "si" translates to "yes" in English, not "oh". (try messing up the "big" words. maybe then no one will notice)
4. Mixed subtitles. If subtitles are in one language, they should all be in that language for the entire show (unless it's a gag in the show). (maybe they should read the instructions on the subtitle program before they use it)