It is a surprising bug to say the least, not too annoying (as you’ll understand, Opera quickly recovers from it), but still, I reported it (DSK-367824).
[Update 2012-06-26: This is a duplicate of DSK-365797]
For no good reason that I can see or that console logs can explain, Opera actions are suddenly mapped to others, regardless that I use their keyboard shortcuts or that I click commands in the menus. When this happen, I feel like Opera is in play mode (like it’s a software thing to replace commands with other actions!).
For example, I want a new tab but get the window to “Open…”. The first time that occurred, I thought “aha! so I need to press cmd-o if I want to get a new tab.” Nope, all I got was the “Save As…” window. In play mode, it’s “Save As…”, or cmd-s, that will open a new tab. Consistently. Except that I don’t know what triggers play mode.
I played as long as I could, it was entertaining. The play-mode key-combos I found are:
cmd-t (for new tab) == open file
cmd-n (for new window) == save-as
cmd-s (to save) == new tab
cmd-c (to copy selection) == fullscreen
cmd-v (to paste selection) == right click
cmd-w (to close tab) == game over, it does cmd-q (to quit).