You have a "stuck" key. The key is not fully stuck, but occasionally kicks in. The most likely key is the "alt", "shift", or "ctrl" on the left side of the keyboard. For example, if "alt" is present at the same time , then a non printing code gets entered.
You could also just not be pressing the key hard enough.
The "sticking" key is a well known problem. Dimples under the key cover cause a capacitance change that is detected as keystroke. The sealing silicon rubber around the edge sometimes overflows slightly into the key area during manufacturing. This is intermittently detected as a capacitance change and hence a keystroke.
I had one such problem with the "alt" key. It started with an occasional miss and then got worse. If a letter did not appear during typing, I would tap the "alt" key several times and then type the desired key again.
My solution was to disassemble and clean off the excess sealant.
One solution reported for maginal cases is warming with a hair dryer (NOT HEATING) the offending key while holding edge next to the key downward (lower edge for "alt" or left edge for "ctrl").
This is very "tricky" and you can make the problem worse.
The ultimate solution is to replace the keyboard -- $18 from ilovemyxo.com .