Most boy teenagers have unexpected erections. Sometimes adult men have unexpected erections. Unexpected erections are involuntary and are normal. People call this a "spontaneous" (sudden or surprise) erection. Sometimes it happens in school. Sometimes it happens walking. Sometimes it happens taking clothes off for a shower or bath or swimming. This is normal. Such erections can be embarrassing if they happen in public. Erections can occur unexpectedly at any time of day.