TIL a level laid on its side on wet mortar will lie to your face

Last Tuesday I was setting the sill course on a small garden wall job out in Glendale and my corners kept creeping out of plumb no matter how careful I was. Took me about 40 minutes of chasing it before I picked up my 4 foot level and noticed mortar smeared across the bottom edge, which was throwing the bubble off by a good 1/8 inch over the length. I scraped it clean with a margin trowel, wiped it down, and checked it against a straight edge, and the wall pulled back in line on the next two courses. Now I wipe the level edges after every few blocks instead of waiting till lunch. Anyone else got a tool that quietly lies to you when it gets dirty?