how to undo a register insertion using i_CTRL-R?

I love the ability of inserting a register content, directly in insert mode, using i_CTRL-R. However, this gets me into trouble from time to time.

For example, I might be on a roll, and without leaving insert mode, I enter a whole paragraph or three, which I want to finish with a contents of a register, which I insert using i_CRTL-R. Except, it turns out I did not get the right register (e.g., there was an intervening cut, so it's no longer "1 but rather "2 or "3). Instead of the expected word or phrase, I accidentally pasted a register which contained massive amount of text. Great, I'll just hit undo key in normal mode to undo it. But hold on! This also undoes the whole paragraph or three I typed in before the mistake.

Gah, how do I undo just the register insertion?! Or do I have to train myself to leave insert mode before register pastes?

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