Superman #135. Telling a story about before Lois was always suspicious of Clark being Superman - a time when his meek timid pose had totally convinced her, so much so that when she caught him changing she was mad at him for trying to fool her...

So, finally she got suspicious enough to try to trap him. She had a small piece of Green Kryptonite and decided to expose him as a test. Fortunately for her, there were plumbers putting new water pipes into the Daily Planet building, so she was able to hide the Green K in a section of pipe next to Clark's office...

Unfortunately for her, the new pipes were lead, so Clark was protected...

That was probably unfortunate for other people as well, though 'they' apparently didn't know this back then...