EXAMPLE

Assume that your lease says you cannot have other people living with you who have not been approved by the landlord.  Your friend moves in and has not been approved by the landlord.  If the landlord finds out about that and lets it go for a while without saying anything and continues accepting your rent, the landlord is waiving that breach. 

If your friend later moves out, the landlord cannot terminate your lease, say, a month after your friend moves out if the landlord knew your friend was living there and permitted him to live there.  A slightly different rule applies for on-going breaches of the lease.

Assume your friend did not move out but continues to live in your unit.  Even though your landlord has waived that breach for a period of time, the landlord has the right to stop waiving the breach and require your friend to leave.  The landlord stops waiving the breach by giving you fair warning in the form of a 14-day notice to have your friend leave the dwelling unit.