should that happens vs should that happen

Some examples from the web:

• Should that happen, it would be inexcusable to the Old Boss.
• Should that happen, the ECB will react without fail by increasing interest would damp down economic growth and employment.
• But should that happen, you may rest assured that the hospital will stand behind you 1 00 per cent.
• Should that happen, the national companies will doubtless continue to be the major operators, but they will have to compete with others on the same terms.
• Should that happen, more upward mobility will be possible, thus negating (2) and (3).
• Should that happen, how would we be able to guarantee the European Union's food security?
• You'll not be here, should that happen?
• Should that happen then, again frequently, an MEP will request verification and as a result the President holds a roll-call vote to that end and the situation is resolved.
• You should've let that happen, man.
• So we should split before that happens.
• So we should split before that happens.
• It should not happen that way.
• He's gathered it up with an armful of oppo, which is what you should do if that happens.
• But I probably should mention little thing that happens at night...
• So you should probably go before that happens.
• So you should probably go before that happens.
• I mean, I shouldn't have let that happen.
• Things got a little heated at the party, and... I shouldn't have let that happen.
• He's going to be cremated first thing in the morning, and I thought he should be gone before that happens.
• I don't think we can say 'should' about things that happen in war.

Some examples from the web:

• But should that happen, you may rest assured that the hospital will stand behind you 1 00 per cent.
• You'll not be here, should that happen?
• Should that happen, it would be inexcusable to the Old Boss.
• Should that happen, the national companies will doubtless continue to be the major operators, but they will have to compete with others on the same terms.
• Should that happen, how would we be able to guarantee the European Union's food security?
• Should that happen then, again frequently, an MEP will request verification and as a result the President holds a roll-call vote to that end and the situation is resolved.
• Should that happen, the ECB will react without fail by increasing interest would damp down economic growth and employment.
• Should that happen, more upward mobility will be possible, thus negating (2) and (3).
• You should've let that happen, man.
• So we should split before that happens.
• So we should split before that happens.
• It should not happen that way.
• He's gathered it up with an armful of oppo, which is what you should do if that happens.
• But I probably should mention little thing that happens at night...
• So you should probably go before that happens.
• So you should probably go before that happens.
• I mean, I shouldn't have let that happen.
• Things got a little heated at the party, and... I shouldn't have let that happen.
• He's going to be cremated first thing in the morning, and I thought he should be gone before that happens.
• I don't think we can say 'should' about things that happen in war.