In math mode, LaTeX will interpret the primed symbolT'asT^{\prime}—i.e., it adds a superscript to theT. When LaTeX subsequently inputs the circumflex character,^, as it reads^{\mu_{1}\mu_{2}\ldots\mu_{q}}, it detects an attempt to add asecond superscriptto theT, and that ...
\boldsymbol \bot \bowtie \Box \boxdot \boxed \boxminus \boxplus \boxtimes \brace \brack \breve \buildrel ... \over ... \bullet \Bumpeq \bumpeq C \cal \Cap \cap \cases \cdot \cdotp \cdots \centerdot \cfrac \check \checkmark ...