I think a big red flag is when a client idolizes a therapist to the point of not accepting that anything COULD be inappropriate. To lack the ability to even "see" the signs or even acknowledge the possibility.

For many, a therapist crossing boundaries feels GOOD. The client loves it... the attention, the feeling "special". The client doesn't want to see or hear it (fingers in ears chanting la la la la la) The client can convince themselves that nothing inappropriate is going on, even when sexually inappropriate behavior is actually going on.

Sadly, it's not uncommon.