Many situations that would produce an error message in other calculators simply create unsimplified formulas in the Emacs Calculator. For example, 1 RET 0 / pushes the formula 1 / 0; 0 L pushes the formula `ln(0)'. Floating-point overflow and underflow are also reasons for this to happen.
When a function call must be left in symbolic form, Calc usually
produces a message explaining why. Messages that are probably
surprising or indicative of user errors are displayed automatically.
Other messages are simply kept in Calc's memory and are displayed only
if you type w (calc-why). You can also press w if
the same computation results in several messages. (The first message
will end with `[w=more]' in this case.)
The d w (calc-auto-why) command controls when error messages
are displayed automatically. (Calc effectively presses w for you
after your computation finishes.) By default, this occurs only for
"important" messages. The other possible modes are to report
all messages automatically, or to report none automatically (so
that you must always press w yourself to see the messages).