What is a big no-no when living in Israel?

Trying to talk politics with an Israeli unprepared. Especially if you don't know what you're talking about.

Giving your boyfriend or husband a kiss after they make pudding in your mouth. It's best to make sure you keep it all in, swallow, then go do chores. He will be shamed by you and want to be alone to pray. 

Ordering a ham and cheese omelette. 

protesting as a left wing or human rights activist

Praying to Mecca.

Upsetting the locals by telling them falafel can't be an Israeli invention because no true Hebrew word begins with an 'f''.  In response to other answers here, yes dressing up like a Nazi would not go down well here, and joking to a security guard that you're carrying a bomb will get you into serious trouble very quickly.  But pork is popular in Tel Aviv, you just call it 'white meat' here.  And visiting Palestinian lands - that's no problem at all, I do it all the time.  Very handy during lockdowns - Israeli restaurants have been shut for ages, but restaurants in Palestinian towns stayed open and lunch is half the price there.

Don't wear German Army surplus shirts (they were popular in the 1990s). I wore one on my first visit to Israel - complete with German flags on the shoulders - and was politely told to remove it.

Not a good idea to open a pork butchers.

Establishing a middle eastern community where there are no borders and everyone can come and go as they wish and live wherever that want..

Pi**ing off the Russian mafia.   If you do it is a good idea to swiftly leave for someplace else.