What it means
'Common error' is MetaTrader's catch-all message when an order fails for a reason it cannot describe more precisely. It tells you the order did not go through, but not exactly why — so you have to investigate.
Common causes
A brief connection drop at the moment you sent the order. A temporary server hiccup. Conflicting or malformed order parameters. An EA sending an invalid request. Or simply a fast market where the request timed out.
Step-by-step fix
Open the Journal tab (or the Experts tab if an EA sent it) and read the lines around the failure — they often contain a more specific reason underneath the generic message.
Check the connection status. Try the order once more after a short pause. Simplify the order (remove S/L and T/P temporarily) to see if a parameter was the issue. If an EA caused it, review its log and inputs.
How to prevent it
Keep a stable connection, avoid firing orders during the most violent seconds of news, and make sure any EA sends valid, well-formed requests. Reading the Journal habitually turns 'Common error' from a mystery into a solvable clue.
Broker's fault or yours?
It can be either. A one-off around news or a connection blip is normal. If it recurs on a good connection in calm markets, the server or your EA is likely at fault — the Journal is the way to tell them apart.