Glass, especially broken glass, is not easily visible. It could represent hidden dangers, or being careless about your direction in life, or a present situation. Being that it was imbedded in your foot could be your subconscious warning you to 'watch your step'...or 'look before you leap'.
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Are you in a situation where you must be careful what you say?
Have you had a disagreement within a relationship recently?
Are you in a situation where you have to make an important decision?
Have you been offered an opportunity of sorts, and you're undecided about whether or not to take it?

Hope that gives you some help. Good Luck!