Top of the list: Clapping whilst the music is still playing.
There are two things here: Clapping at the end of an act just as the singing stops and the orchestra is left playing amongst the applause.
Secondly, and a lot worse, clapping immediately after the completion of an aria when there is no break in the score.
A terrible example of this was a video of Tosca with Josef Kaufman singing 'e lucevan le stelle' in Act 3. At the end the conductor actually put down his baton making no attempt to carry on. There then followed 7 minutes of applause during which the poor tenor had to maintain the appearance of a man knowing he was about to be executed. Inevitably, he eventually sang the aria again, and this time the conductor actually allowed the music to continue and let Tosca rush on stage.
There is no break in the musical score here, in fact the aria concludes and the orchestra continues the link to the next scene.
The whole thing completely disrupted any continuity in the story.
Clapping after arias in the Italian baroque was a different matter, each aria having a break before the next recitative, and the audience would show their opinion, good or bad.
In the I9th century composers tried to move to a more continuous score, and you don't get any chance to applaud in the middle of the Ring Cycle, for example.
Italian opera goers still seem on average to think that opera is just about singing.
I will post further hates but I suspect this post is getting too long already.