Eh, from a communication standpoint, it's one that throws me off a little bit more than other mistakes when I am talking to an L2 speaker...largely because there is a larger difference in meaning than some other run-of-the-mill grammatical errors. If we are talking about a woman, and the other speaker starts mentioning 'he' and 'him', I will get confuse about the mystery man, until I realize what is going on. If someone doesn't make the subject agree with the verb--it's a pretty obvious mistake that I understand right away. Not sure if it is the most common, though.  

As long as we're communicating, I can't really judge people for their English errors. It's not like my Chinese is perfect.

Yes! It's insane, I will speak to people whose English is quite impressive, who use choice scholarly words yet their inability to use the correct he/she form drives me insane. I'm forever correcting people.

i usually ignore it as best i can, outside of the classroom, so long as i can follow the conversation. But there are times i am totally confused about who the speaker is talking about. Having learnt languages where there are masculine and feminine nouns and word endings, it can be quite irritating that something as simple as this is a common error.

I'm just going to give this question a like. Simple as that.

one of the things that surprise me with chinese is that they are very lazy with english

chinese requires so much effort but they do not give the same respect to english.

but i am beginning to think they are lazy with chinese as well

To me, it is not only gender, but using wrongly singular vs plural, and tense of verbs.  It is so bad sometimes, that I totally was not able to understand most of the conversations.

Yet, there is one that I will try to correct anymore. He dared to say that he spoke better English than me, because he learned English listening to American TV shows. Patience !

my real life example or this...

Was out with a good friend and a friend of hers. The friends friend kept talking about how great her "English speak" was. I neither confirmed nor denied this (I let people think however they want). She started talking about someone at the bar and kept using he. I had to stop her mid-story to figure out which "he" she was referring to. I could only see women. She says "him right there in the red dress". I promise I wasn't trying to sound condescending but I asked her "are you talking about the lady? "She" is the only one with the red dress". This girl then proceeded to tell me how bad my English understanding was and I should listen better.

Can't win for losing...

He told me to go out....She said...that's just being direct my friend...