The equation editor seems to be glitching.
The problem seems to occur while attempting to type an expression such as:
1/2 U"(w_t)
1/2 changes into a fraction fine. The expression within U"( ) does not become w subscript t.
When the equation becomes "corrupted" with this error, clearing its contents and attempting to type anything else (for example, x^2 does not change it to x power 2).
Is equation editor perhaps interpreting one of the expressions I've typed in as something else that I'm aware of?