Block switching?

Ok so I was making a rainbow themed base with the new concrete blocks and i was placing a bunch of white ones all in a line and for some reason the game will randomly glitch out and instead of placing the intended block it places a purple glass pane.
Just before that happened i was setting up a beacon with a rainbow theme, I'm not sure if that had something to do with it or not.
Its not a particularly infuriating bug, but i thought it should be mentioned.

EDIT:
This was not quite the problem i thought it was, when i opened up my game again after exiting the world, it turned out purple glass panes were in my off hand. This wasnt showing up earlier though so i guess thats the actual problem. Sorry about being confusing, mojang.