I cannot insert carets and backticks (and possibly other characters) in XTerm under certain conditions. When I move focus from XTerm to Firefox and use Alt+Tab to switch back, I can no longer instert ^ and `.
Strangely enough, I can only reproduce the problem by Alt+Tabbing from Firefox to XTerm, other applications don't seem to cause the issue. From what I've tried, it really is the act of tabbing into the XTerm, tabbing to Firefox and clicking back works just normally.
I am running XTerm(297) on Ubuntu 14.04.2 LTS. I'm using a German keyboard layout, so I have to follow carets and backticks by a space to make them appear alone. With this issue, pressing the backtick/caret key does simply nothing at all, and the following space inserts a space. When I use a US keyboard layout for testing, everything works fine too.
Please help me solve this mystery.