diff --git a/README.md b/README.md
index 979f71e6..38c4d8a0 100644
--- a/README.md
+++ b/README.md
@@ -6,13 +6,13 @@
 Cron V3 has been released!
 
 To download the specific tagged release, run:
-
-	go get github.com/robfig/cron/v3@v3.0.0
-
+```bash
+go get github.com/robfig/cron/v3@v3.0.0
+```
 Import it in your program as:
-
-	import "github.com/robfig/cron/v3"
-
+```go
+import "github.com/robfig/cron/v3"
+```
 It requires Go 1.11 or later due to usage of Go Modules.
 
 Refer to the documentation here:
@@ -65,15 +65,15 @@ It is backwards incompatible with both v1 and v2. These updates are required:
 
   UPDATING: To retain the old behavior, construct your Cron with a custom
   parser:
-
-      // Seconds field, required
-      cron.New(cron.WithSeconds())
-
-      // Seconds field, optional
-      cron.New(
-          cron.WithParser(
-              cron.SecondOptional | cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor))
-
+```go
+// Seconds field, required
+cron.New(cron.WithSeconds())
+
+// Seconds field, optional
+cron.New(cron.WithParser(cron.NewParser(
+	cron.SecondOptional | cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor,
+)))
+```
 - The Cron type now accepts functional options on construction rather than the
   previous ad-hoc behavior modification mechanisms (setting a field, calling a setter).
 
@@ -92,20 +92,20 @@ It is backwards incompatible with both v1 and v2. These updates are required:
   has been removed to accommodate the more general JobWrapper type.
 
   UPDATING: To opt into panic recovery and configure the panic logger:
-
-      cron.New(cron.WithChain(
-          cron.Recover(logger),  // or use cron.DefaultLogger
-      ))
-
+```go
+cron.New(cron.WithChain(
+  cron.Recover(logger),  // or use cron.DefaultLogger
+))
+```
 - In adding support for https://github.com/go-logr/logr, `cron.WithVerboseLogger` was
   removed, since it is duplicative with the leveled logging.
 
   UPDATING: Callers should use `WithLogger` and specify a logger that does not
   discard `Info` logs. For convenience, one is provided that wraps `*log.Logger`:
-
-      cron.New(
-          cron.WithLogger(cron.VerbosePrintfLogger(logger)))
-
+```go
+cron.New(
+  cron.WithLogger(cron.VerbosePrintfLogger(logger)))
+```
 
 ### Background - Cron spec format
 
@@ -118,7 +118,7 @@ There are two cron spec formats in common usage:
   jobs in Java software
 
 [the Cron wikipedia page]: https://en.wikipedia.org/wiki/Cron
-[the Quartz Scheduler]: http://www.quartz-scheduler.org/documentation/quartz-2.x/tutorials/crontrigger.html
+[the Quartz Scheduler]: http://www.quartz-scheduler.org/documentation/quartz-2.3.0/tutorials/tutorial-lesson-06.html
 
 The original version of this package included an optional "seconds" field, which
 made it incompatible with both of these formats. Now, the "standard" format is
diff --git a/chain.go b/chain.go
index 118e5bbe..9c087b7b 100644
--- a/chain.go
+++ b/chain.go
@@ -76,14 +76,14 @@ func DelayIfStillRunning(logger Logger) JobWrapper {
 // SkipIfStillRunning skips an invocation of the Job if a previous invocation is
 // still running. It logs skips to the given logger at Info level.
 func SkipIfStillRunning(logger Logger) JobWrapper {
-	var ch = make(chan struct{}, 1)
-	ch <- struct{}{}
 	return func(j Job) Job {
+		var ch = make(chan struct{}, 1)
+		ch <- struct{}{}
 		return FuncJob(func() {
 			select {
 			case v := <-ch:
+				defer func() { ch <- v }()
 				j.Run()
-				ch <- v
 			default:
 				logger.Info("skip")
 			}
diff --git a/chain_test.go b/chain_test.go
index 2561bd7f..ec910975 100644
--- a/chain_test.go
+++ b/chain_test.go
@@ -218,4 +218,25 @@ func TestChainSkipIfStillRunning(t *testing.T) {
 		}
 	})
 
+	t.Run("different jobs independent", func(t *testing.T) {
+		var j1, j2 countJob
+		j1.delay = 10 * time.Millisecond
+		j2.delay = 10 * time.Millisecond
+		chain := NewChain(SkipIfStillRunning(DiscardLogger))
+		wrappedJob1 := chain.Then(&j1)
+		wrappedJob2 := chain.Then(&j2)
+		for i := 0; i < 11; i++ {
+			go wrappedJob1.Run()
+			go wrappedJob2.Run()
+		}
+		time.Sleep(100 * time.Millisecond)
+		var (
+			done1 = j1.Done()
+			done2 = j2.Done()
+		)
+		if done1 != 1 || done2 != 1 {
+			t.Error("expected both jobs executed once, got", done1, "and", done2)
+		}
+	})
+
 }
diff --git a/cron.go b/cron.go
index f6e451db..c7e91766 100644
--- a/cron.go
+++ b/cron.go
@@ -21,11 +21,16 @@ type Cron struct {
 	logger    Logger
 	runningMu sync.Mutex
 	location  *time.Location
-	parser    Parser
+	parser    ScheduleParser
 	nextID    EntryID
 	jobWaiter sync.WaitGroup
 }
 
+// ScheduleParser is an interface for schedule spec parsers that return a Schedule
+type ScheduleParser interface {
+	Parse(spec string) (Schedule, error)
+}
+
 // Job is an interface for submitted cron jobs.
 type Job interface {
 	Run()
diff --git a/doc.go b/doc.go
index aa5a76f2..fa5d08b4 100644
--- a/doc.go
+++ b/doc.go
@@ -21,7 +21,7 @@ them in their own goroutines.
 	c := cron.New()
 	c.AddFunc("30 * * * *", func() { fmt.Println("Every hour on the half hour") })
 	c.AddFunc("30 3-6,20-23 * * *", func() { fmt.Println(".. in the range 3-6am, 8-11pm") })
-	c.AddFunc("CRON_TZ=Asia/Tokyo 30 04 * * * *", func() { fmt.Println("Runs at 04:30 Tokyo time every day") })
+	c.AddFunc("CRON_TZ=Asia/Tokyo 30 04 * * *", func() { fmt.Println("Runs at 04:30 Tokyo time every day") })
 	c.AddFunc("@hourly",      func() { fmt.Println("Every hour, starting an hour from now") })
 	c.AddFunc("@every 1h30m", func() { fmt.Println("Every hour thirty, starting an hour thirty from now") })
 	c.Start()
diff --git a/option.go b/option.go
index 07638201..09e4278e 100644
--- a/option.go
+++ b/option.go
@@ -23,7 +23,7 @@ func WithSeconds() Option {
 }
 
 // WithParser overrides the parser used for interpreting job schedules.
-func WithParser(p Parser) Option {
+func WithParser(p ScheduleParser) Option {
 	return func(c *Cron) {
 		c.parser = p
 	}
diff --git a/parser.go b/parser.go
index 3cf8879f..9a294fba 100644
--- a/parser.go
+++ b/parser.go
@@ -61,11 +61,11 @@ type Parser struct {
 //  sched, err := specParser.Parse("0 0 15 */3 *")
 //
 //  // Same as above, just excludes time fields
-//  subsParser := NewParser(Dom | Month | Dow)
+//  specParser := NewParser(Dom | Month | Dow)
 //  sched, err := specParser.Parse("15 */3 *")
 //
 //  // Same as above, just makes Dow optional
-//  subsParser := NewParser(Dom | Month | DowOptional)
+//  specParser := NewParser(Dom | Month | DowOptional)
 //  sched, err := specParser.Parse("15 */3")
 //
 func NewParser(options ParseOption) Parser {
@@ -313,6 +313,9 @@ func getRange(expr string, r bounds) (uint64, error) {
 	if step == 0 {
 		return 0, fmt.Errorf("step of range should be a positive number: %s", expr)
 	}
+	if step > r.max-r.min {
+		return 0, fmt.Errorf("step (%d) above maximum (%d): %s", step, r.max-r.min, expr)
+	}
 
 	return getBits(start, end, step) | extra, nil
 }
diff --git a/parser_test.go b/parser_test.go
index 41c8c520..8d48a46b 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -334,6 +334,18 @@ func TestStandardSpecSchedule(t *testing.T) {
 			expr: "* * * *",
 			err:  "expected exactly 5 fields",
 		},
+		{
+			// Step value larger than the field range should be rejected.
+			// */90 in the minute field (range 0-59) would only ever fire at
+			// minute 0, silently ignoring the intent.  See issue #543.
+			expr: "*/90 * * * *",
+			err:  "step (90) above maximum (59)",
+		},
+		{
+			// Same check for hours: */30 is the max valid step (0-23 → range 23).
+			expr: "* */30 * * *",
+			err:  "step (30) above maximum (23)",
+		},
 	}
 
 	for _, c := range entries {