diff --git a/client/src/components/hints.tsx b/client/src/components/hints.tsx
index bec14b4..10aa46f 100644
--- a/client/src/components/hints.tsx
+++ b/client/src/components/hints.tsx
@@ -8,7 +8,7 @@ export function Hint({hint, step, selected, toggleSelection} : {hint: GameHint,
   </div>
 }
 
-export function AdditionalHint({hint, step, selected, toggleSelection} : {hint: GameHint, step: number, selected: number, toggleSelection: any}) {
+export function HiddenHint({hint, step, selected, toggleSelection} : {hint: GameHint, step: number, selected: number, toggleSelection: any}) {
   return <div className={`message warning step-${step}` + (step == selected ? ' selected' : '')} onClick={toggleSelection}>
     <Markdown>{hint.text}</Markdown>
   </div>
@@ -19,8 +19,9 @@ export function Hints({hints, showHidden, step, selected, toggleSelection} : {hi
   const openHints = hints.filter(hint => !hint.hidden)
   const hiddenHints = hints.filter(hint => hint.hidden)
 
+  // TODO: Should not use index as key.
   return <>
-    {openHints.map(hint => <Hint hint={hint} step={step} selected={selected} toggleSelection={toggleSelection}/>)}
-    {showHidden && hiddenHints.map(hint => <AdditionalHint hint={hint} step={step} selected={selected} toggleSelection={toggleSelection}/>)}
+    {openHints.map((hint, j) => <Hint key={`hint-${step}-${j}`} hint={hint} step={step} selected={selected} toggleSelection={toggleSelection}/>)}
+    {showHidden && hiddenHints.map((hint, j) => <HiddenHint key={`hidden-hint-${step}-${j}`} hint={hint} step={step} selected={selected} toggleSelection={toggleSelection}/>)}
   </>
 }
diff --git a/client/src/components/infoview/main.tsx b/client/src/components/infoview/main.tsx
index 3c3f21e..55fabf9 100644
--- a/client/src/components/infoview/main.tsx
+++ b/client/src/components/infoview/main.tsx
@@ -255,7 +255,8 @@ function GoalsTab({proofStep} : {proofStep: ProofStep}) {
   return <div>
     <div className="tab-bar">
       {proofStep.goals.map((goal, i) => (
-        <div className={`tab ${i == (selectedGoal) ? "active": ""}`} onClick={(ev) => { setSelectedGoal(i); ev.stopPropagation() }}>
+        // TODO: Should not use index as key.
+        <div key={`proof-goal-${i}`} className={`tab ${i == (selectedGoal) ? "active": ""}`} onClick={(ev) => { setSelectedGoal(i); ev.stopPropagation() }}>
           {i ? `Goal ${i+1}` : "Active Goal"}
         </div>
       ))}
@@ -382,16 +383,15 @@ export function CommandLineInterface(props: {world: string, level: number, data:
         if (i == proof.length - 1 && hasInteractiveErrors(step.errors)) {
           // if the last command contains an error, we only display the errors but not the
           // entered command as it is still present in the command line.
-          return <div>
+          // TODO: Should not use index as key.
+          return <div key={`proof-step-${i}`}>
             <Errors errors={step.errors} commandLineMode={true}/>
           </div>
         } else {
-          return <>
-            <div className={`step step-${i}` + (selectedStep == i ? ' selected' : '')} onClick={toggleSelectStep(i)}>
-              <Command command={step.command} deleteProof={deleteProof(i)}/>
-              <Errors errors={step.errors} commandLineMode={true}/>
-              <GoalsTab proofStep={step}/>
-            </div>
+          return <div key={`proof-step-${i}`} className={`step step-${i}` + (selectedStep == i ? ' selected' : '')} onClick={toggleSelectStep(i)}>
+            <Command command={step.command} deleteProof={deleteProof(i)}/>
+            <Errors errors={step.errors} commandLineMode={true}/>
+            <GoalsTab proofStep={step}/>
             {/* Show a message that there are no goals left */}
             {!step.goals.length && (
               <div className="message information">
@@ -404,7 +404,7 @@ export function CommandLineInterface(props: {world: string, level: number, data:
                 }
               </div>
             )}
-          </>
+          </div>
         }
       }) : <CircularProgress />}
     </div>
diff --git a/client/src/components/infoview/messages.tsx b/client/src/components/infoview/messages.tsx
index f010fd1..fa88f33 100644
--- a/client/src/components/infoview/messages.tsx
+++ b/client/src/components/infoview/messages.tsx
@@ -48,10 +48,11 @@ function Error({error, commandLineMode} : {error : InteractiveDiagnostic, comman
   </div>
 }
 
+// TODO: Should not use index as key.
 /** A list of messages (info/warning/error) that are produced after this command */
 export function Errors ({errors, commandLineMode} : {errors : InteractiveDiagnostic[], commandLineMode : boolean}) {
   return <div>
-    {errors.map((err) => (<Error error={err} commandLineMode={commandLineMode}/>))}
+    {errors.map((err, i) => (<Error key={`error-${i}`} error={err} commandLineMode={commandLineMode}/>))}
   </div>
 }
 
diff --git a/client/src/components/inventory.tsx b/client/src/components/inventory.tsx
index e51e07a..1c815bd 100644
--- a/client/src/components/inventory.tsx
+++ b/client/src/components/inventory.tsx
@@ -60,20 +60,21 @@ function InventoryList({items, docType, openDoc, defaultTab=null, level=undefine
     {categories.length > 1 &&
       <div className="tab-bar">
         {categories.map((cat) =>
-          <div className={`tab ${cat == (tab ?? categories[0]) ? "active": ""}`} onClick={() => { setTab(cat) }}>{cat}</div>)}
+          <div key={`category-${cat}`} className={`tab ${cat == (tab ?? categories[0]) ? "active": ""}`}
+            onClick={() => { setTab(cat) }}>{cat}</div>)}
       </div>}
     <div className="inventory-list">
-    { [...items].sort(
-      // For lemas, sort entries `available > disabled > locked`
-      // otherwise alphabetically
-      (x, y) => +(docType == "Lemma") * (+x.locked - +y.locked || +x.disabled - +y.disabled)
-    ).map(item => {
-        if ((tab ?? categories[0]) == item.category) {
-          return <InventoryItem key={item.name} showDoc={() => {openDoc(item.name, docType)}}
+    {[...items].sort(
+        // For lemas, sort entries `available > disabled > locked`
+        // otherwise alphabetically
+        (x, y) => +(docType == "Lemma") * (+x.locked - +y.locked || +x.disabled - +y.disabled)
+      ).filter(item => ((tab ?? categories[0]) == item.category)).map((item, i) => {
+          return <InventoryItem key={`${item.category}-${item.name}`}
+            showDoc={() => {openDoc(item.name, docType)}}
             name={item.name} displayName={item.displayName} locked={item.locked}
             disabled={item.disabled} newly={item.new}/>
-        }
-      }) }
+      })
+    }
     </div>
     </>
 }
diff --git a/client/src/components/level.tsx b/client/src/components/level.tsx
index 2b5c604..aace273 100644
--- a/client/src/components/level.tsx
+++ b/client/src/components/level.tsx
@@ -230,8 +230,10 @@ function PlayableLevel({worldId, levelId}) {
               // It the last step has errors, it will have the same hints
               // as the second-to-last step. Therefore we should not display them.
               if (!(i == proof.length - 1 && hasInteractiveErrors(step.errors))) {
-                return <Hints hints={step.hints} showHidden={showHiddenHints} step={i}
-                selected={selectedStep} toggleSelection={toggleSelection(i)}/>
+                // TODO: Should not use index as key.
+                return <Hints key={`hints-${i}`}
+                  hints={step.hints} showHidden={showHiddenHints} step={i}
+                  selected={selectedStep} toggleSelection={toggleSelection(i)}/>
               }
             })}
             {completed &&