There was the time I spent an entire day trying to make electrophoretic gels using distilled water instead of buffer. The agarose suspended fine in the distilled water, but I couldn’t get it to solidify. I kept remaking the agarose solution and never got anything worthwhile. The next day, I asked a lab mate what I was doing wrong. I found out and felt damn stupid.
And just last week the autoclave leaked. A brown liquid covered the floor. It was real funny while I cleaned it up.