I was curious about this. Google tells me that aircraft seating charts typically end on an even number and try to have a standardized seating map, so some newer Boeing aircraft skip row 33 so it can end on an even number.
Row 33 is missing on the 757-200, 737-700, 737-800, 737-900, A319, and A320. They are actually missing rows at other areas of the plane, too often 5-6 and 13-19 are skipped, depending on the class/section configuration.
If you're on United, all United aircraft have exit rows at rows 20 and 21.
Skipping rows seems to be a situation most likely to occur on United planes (but not exclusively).
Also sounds like they may stop doing this in the near future.