It can even be funny. Consider the unintended humour in these signs (all seen by me in various places around the world)...
- No standing at any time (a no parking sign)
- This door is alarmed
- Do not use lift (elevator) in case of fire
- Cyclists use left shoulder
The only way to avoid such mistakes, is to have several people check your copy before it goes. And these people must be in your target market. That way, you avoid making a mistake through ignorance (e.g. not knowing a particular word has a different meaning among your target audience).
You'll also help your cause if you make sure every sentence you write contains only one sentence. Keep your eyes peeled for sentences that feature semi colons, or more than 2 commas.