layer opacity changes automatically?

[Joost]

I noticed today that my layer opacity is changing, although I think I didn't do anything to change it.

Only the layers where I work in that don'd have a 100 percent opacity change (from 21% to 20 %). It happened several times today, but I only noticed it after it happened. Am I maybe accidentally using a keyboard shortcut that can decrease my opacity with one percent?

[CLO]

That's strange. I can't find any hotkeys that allow to lower the opacity.