Why does this happen when I type sometimes (not on fluther)?

An example is when I’m typing on my online college discussion boards. For some reason, seemingly random when I try to correct a spelling error by simply inserting a letter in a word, it will delete the ahead of the cursor when I hit the space bar to insert the letter. It’s as if the space bar is the delete key.

And simply putting the cursor in the middle of a word to insert a letter just deletes the letter infront of the cursor although it still inserts the letter of the key I press.

Is this making sense? Does anyone know how this happens?