Why you do not want to use `render*()` inside `observeEvent()`: it does not do what you think it does

render_and_observeEvent.R

library(shiny)

# Define UI for application that draws a histogram
ui <- fluidPage(

    # Application title
    titlePanel("Old Faithful Geyser Data"),

    # Sidebar with a slider input for number of bins
    sidebarLayout(
        sidebarPanel(
            sliderInput("bins",
                        "Number of bins:",
                        min = 1,
                        max = 50,
                        value = 30)
        ),

        # Show a plot of the generated distribution
        mainPanel(
           plotOutput("distPlot")
        )
    )
)

# Define server logic required to draw a histogram
server <- function(input, output) {

  local <- reactiveValues(val = 0)

  observeEvent(input$bins, {

    output$distPlot <- renderPlot({
        # generate bins based on input$bins from ui.R
        x    <- faithful[, 2]
        bins <- seq(min(x), max(x), length.out = input$bins + 1)

        showNotification(paste("Message", local$val), duration = 2)
        isolate({local$val <- local$val + 1})
        # draw the histogram with the specified number of bins
        hist(x, breaks = bins, col = 'darkgray', border = 'white')
    })
  })
}

shinyApp(ui = ui, server = server)

Here, observeEvent() does not prevent renderPlot() to be updated each time loacl$val has changed, although you wanted to only trigger the code when input$bins has changed