There are a few different synonyms that could be used in place of "offer a word of caution." For example, one might "warn" someone about a potential danger or issue, or "advise" them to be careful or cautious. Another option could be to "alert" someone to a potential problem or issue that they may not have considered. Some other alternatives might include "caution," "remind," "point out," or "suggest." Ultimately, the choice of word will depend on the context and tone of the conversation, as well as the speaker's relationship with the person they are cautioning.