If you make a mistake in an equation,
like leaving out a brace (very common)
or having one too many (very common)
or having a
with nothing before it (common),
will tell you with the message
syntax error between lines x and y, file z
where x and y are approximately the lines between which the trouble occurred, and z is the name of the file in question. The line numbers are approximate _ look nearby as well. There are also self-explanatory messages that arise if you leave out a quote or try to run EQN on a non-existent file.
If you want to check a document before actually printing it
eqn files >/dev/null
will throw away the output but print the messages.
If you use something like dollar signs as delimiters, it is easy to leave one out. This causes very strange troubles. The program checkeq (on GCOS, use ./checkeq instead) checks for misplaced or missing dollar signs and similar troubles.
In-line equations can only be so big because of an internal buffer in TROFF. If you get a message ``word overflow'', you have exceeded this limit. If you print the equation as a displayed equation this message will usually go away. The message ``line overflow'' indicates you have exceeded an even bigger buffer. The only cure for this is to break the equation into two separate ones.
On a related topic, EQN does not break equations by itself _ you must split long equations up across multiple lines by yourself, marking each by a separate .EQ ... .EN sequence. EQN does warn about equations that are too long to fit on one line.