`rm(list = ls())` probably doesn't do what you think it does

blank-slate-problem.R

filter(head(presidents), rep(1, 3))  # this uses utils::filter
#> Time Series:
#> Start = 1 
#> End = 6 
#> Frequency = 1 
#> [1]  NA  NA 244 220 188  NA

library(dplyr, warn.conflicts = FALSE)  # attach dplyr

search()  # shows dplyr will be preferred to utils if there's a nameclash
#>  [1] ".GlobalEnv"        "package:dplyr"     "package:stats"    
#>  [4] "package:graphics"  "package:grDevices" "package:utils"    
#>  [7] "package:datasets"  "package:methods"   "Autoloads"        
#> [10] "tools:callr"       "package:base"

filter(head(presidents), rep(1, 3))  # defaults now to dplyr::filter, so will error
#> Error in UseMethod("filter"): no applicable method for 'filter' applied to an object of class "c('double', 'numeric')"

rm(list = ls())  # 'clear workspace', detaches dplyr?

filter(head(presidents), rep(1, 3))  # uhoh, it still fails!
#> Error in UseMethod("filter"): no applicable method for 'filter' applied to an object of class "c('double', 'numeric')"

search()  # it's because rm() didn't detach dplyr, so dplyr::filter was used
#>  [1] ".GlobalEnv"        "package:dplyr"     "package:stats"    
#>  [4] "package:graphics"  "package:grDevices" "package:utils"    
#>  [7] "package:datasets"  "package:methods"   "Autoloads"        
#> [10] "tools:callr"       "package:base"

# A full restart of R will do the job (Session > Restart R, or Ctrl/Cmd+Shift+F10)